WHAT HAPPENED IN THE SECOND SHINOBI WORLD WAR?
The Second Shinobi World War is fought between Konohagakure, led by the Third Hokage, Sunagakure, likely led by the Third Kazekage, Amegakure, led by Hanzō of the Salamander, and Iwagakure, presumably led by the Third Tsuchikage. It means Kumogakure and Kirigakure never fought with Konohagakure in this Shinobi World War unlike the Third Shinobi World War. So this raise a question: Why would a power-hungry village like Kumogakure and a bloodthirsty village like Kirigakure will keep themselves out of conflicts? The answer in my opinion is: They fought and destroyed Uzushiogakure in this war but in doing so suffered massive casualties which weakened them so much that out of self-preservation, it left them with no choice but to withdraw from the war and keep themselves out of further conflicts. Hence why they didn't fought against Konohagakure in the Second Shinobi World War whereas in the Third Shinobi World War, both villages did fought against Konohagakure.
*And we have proof that the destruction of Uzushiogakure took place during the Second Shinobi World War. Because according to "Naruto chapter 500, page 4"; the destruction of the Uzushiogakure occurred during Kushina's lifetime, either shortly before or just after she moved to Konohagakure. And Kushina Uzumaki arrived in Konohagakure before the start of the Third Shinobi World War.
*Nagato in "Naruto chapter 445, page 3" referred to 'The Second Shinobi World War' as a war Konoha started. It means there must have been something extreme done against Konohagakure by some other shinobi village otherwise the chances of a Shinobi World War started by the ever peaceful Hiruzen Sarutobi is slim to none.
*During this war Konohagakure was responsible for the destruction of The Land of the Sky. For the ever peaceful Hiruzen Sarutobi to agree to do such a thing against an enemy village, it means the hidden village of the Land of the Sky must have done something heinous against Konohagakure to warrant such an action. Something like the majority of the Senju Clan being ambushed and wiped out by the Land of the Sky forces.(Remember in the Naruto movie when Tsunade heard about them, she was terrified, this must have been the reason.)
*Why did The Land of the Sky attacked Konohagakure?
After Konohagakure's victory over the other villages during the First Shinobi World War, they were pretty much regarded as the strongest hidden village as such in a bid to gain fame, the newly formed hidden village of 'The Land of the Sky' launched an invasion on the strongest hidden village of that era and attacked the strongest clan of the world, namely Senju Clan.
*How the attack took place?
To celebrate his son's marriage, the Fire Daimyō organised a feast in a newly built luxurious hotel in the Land of Fire and invited the majority of the Senju Clan, the Third Hokage and other important Clan Heads, dignitaries, nobles, council members etc. Unfortunately, somehow this information was leaked to the Land of the Sky who were already planning to attack the Land of Fire and Konohagakure, prompting them to accelerate their plan and they decided to attack on this particular day. As expected all of those who were invited by the Fire Damiyō attended the party and since the party was organised in the heart of the Land of Fire nobody ever expected any kind of attacks much less a large scale aerial invasion. The attack took place and the Konohagakure forces were caught off-guard, having no experience in dealing with aerial threats much less one on such a large scale and surprise being a key factor in their enemy's side, the Land of the Sky forces massacred the Senju Clan with heavy aerial bombardment. Still before going down the Senju and Konohagakure forces managed to safeguard the Fire Daimyō, the Third Hokage, other Konoha Clan Heads, council members, dignitaries, nobles, hotel staff and the citizens of the Land of Fire.
*How did the Shinobi World and Konohagakure reacted?
As expected the Third Hokage declared war on the Land of the Sky for this unprovoked attack. Seeing this as an opportunity Sunagakure attacked the Land of Fire and tried to pressurise the weakened Konohagakure in handing over the fertile piece of the Land granted to them for farming by the First Hokage. To cure their economic disparity, hidden villages such as Iwagakure, Takigakure, etc who were already looking for an opportunity to expand their territories using military force but were cowed due to fear of retaliation from Konohagakure. So the attack by the hidden village of the Land of the Sky and Sunagakure on Konohagakure gave rise to an opportunity which these villages fully intended to utilise and thus under the pretext of expending fair rights, these countries started to expand. As expected Konohagakure's allies asked for help and protection hence Konoha had no choice but to respond, starting the Second Shinobi World War. Eventually Land of Rain is started to be used as a battlefield leaving Amegakure with no choice but to retaliate.
Kumogakure and Kirigakure too saw this as an opportunity and started raiding the ports of the Land of Fire prompting Konohagakure to ask for the help of their ally Uzushiogakure who agreed and started defending the shorelines of the Land of Fire against the attacks from Kumogakure and Kirigakure. During these skirmishes between Kiri, Kumo and Uzu Shinobi, some high ranking shinobi from both Kumogakure and Kirigakure insulted Ashina Uzumaki on different ocassion saying how the Senju Clan and all of the three Hokages had always and will always view the Uzumaki Clan and Uzushiogakure as nothing more than their loyal pet dogs, being kept in cages when they aren't throwing them at their enemies and, always had and will forever treat them as nothing more than trash and dirt. How he had allowed the Uzumaki Clan and Uzushiogakure to be manipulated, used and taken advantage off by Konohagakure. How Konohagakure had numerous times trampled on the dignity of Uzushiogakure and Uzumaki Clan and had stabbed them in the back be it during the Bijū distribution, trade policies, etc. Despite all of these treacherous actions of Konohagakure how Ashina had done nothing but bow to Konoha's demands like a good loyal puppy does for their master.
In response Ashina killed all of them and later on during face-offs with the Raikage and Mizukage, to vent off his frustrations, he insulted the both of them on different occasions saying how he is greater than any Kage or other village leaders because unlike them he is subservient to no one, not even to a Daimyō. That he is the sole authority of not only Uzushiogakure but also the Land of Whirlpools. He then further insulted them by saying how they were the original trained pet dogs in the eyes of their respective Daimyō and the rest of the Shinobi World. How they had never been considered an equal by their respective Daimyōs and never would unlike the Hokage. Unfortunately, Ashina's audacity was seen as a grave insult to both the Raikage and the Mizukage. It irked the both of them in the wrong way. Some shinobi from Kiri and Kumo present during these face-offs with Ashina, in a bid to gain more favour from their lord Daimyōs, informed them on how Ashina Uzumaki was trying to goad their respective Kages to rebel against the Daimyōs.
The Water Daimyō knew how bloodthirsty the entire village of Kirigakure is and how big of a madman their Third Mizukage is, so he too knew that it was only a matter of time before the constant goading would made them explode and, rebel against him and plunge the Land of Water into a civil war. And knowing the prowess of the Kirigakure's Shinobi, he knew there is a chance that they might also succeed in overthrowing him as well. It was a grave concern for him. The mere thought of facing Kirigakure terrified him to no end.
As a result of this experience, all of the future Water Daimyōs will forever favour policies that will never allow Kirigakure to remain united as one and, will made the natives of the Land of Water to view shinobi as nothing more than bloodthirsty demons and him as their only saviour. These policies favoured by the Daimyō and followed by Mizukage will always favour some clans over the others keeping them forever disunited and will eventually led them to the point that the citizens of Kirigakure will start to openly discriminate the Kekkei Genkei bearing clans. It will then form a non-bloodline faction and bloodline faction in their village, starting a bitter and bloodthirsty rivalry between them, the non-bloodline faction will eventually start committing genocide of all of those with Kekkei Genkai due to receiving the full support of their government. This will led to the formation of the Bloody Mist Village, and then to the eventual 'Civil War'.
The Water Daimyō knew he had to do something quickly to channel the bloodlust of that Mizukage and Kirigakure's shinobi somewhere else to save himself. And at that moment a sinister plan formed in his mind, a plan which will not only save his lordship but aslo benifit him and increase the territory of the Land of Water a lot more. It will also punish that bastard Ashina and those Uzumakis for messing with him, and trying to rile up the Mizukage against him. He knew that that bastard Ashina had also tried to play the same trick with the Third Raikage. He will write to the Lightning Daimyō and will make sure that Ashina knows that he is messing up with the Daimyōs of two of the Five Great Nations.
Meanwhile with the Lightning Daimyō, he too was worried about the actions of Ashina Uzumaki. The Lightning Daimyō was aware of the hot-headed, prideful and power-hungry nature of the Third Raikage; he knew the constant goading could also potentially lead their countries to a civil war, the danger is there although the chances of it happening is slim to none but nonetheless the chances are there. And it worried him to no end. He had received the message from the Water Daimyō regarding on launching a joint invasion on the Land of Whirlpools to teach Ashina and the village of Uzushiogakure a lesson. Although the reasons given by the Water Daimyō are valid since the existence of a land solely ruled by a Shinobi Clan is a dangerous existence, it could fill the head of shinobis especially Kages with the stupid idea of also becoming the sole ruler of their own land and not being subservient to their Daimyōs but he had seen right through the plan of the Water Daimyō. The Land of Whirlpools is the only resistance that have stopped the Land of Waters from expanding further in the Central Elemental Sea. With them gone and Konohagakure having little to no experience in Sea Warfare, that bastard will be able to easily expand and further assimilate the Land of Sea, Land of Waves and Land of Whirlpools into the territory of Land of Waters. He won't do something as stupid as agree with this plan of jointly invading Uzushiogakure and waste his resources while that bastard Water Daimyō gains more. Thus the Lightning Daimyō rejected the plan and the destruction of Uzushiogakure is averted.
But then something happened that would change the course of war: Ashina captured the wife of Third Raikage. And out of frustrations and to show the world that he and Uzushiogakure can do whatever they want, he executed her despite facing severe opposition from Konohagakure who tried by all means to convince him not to do such a stupid thing, but Konohagakure's pleas fell on deaf ears. The Raikage swore that he will make the entire Land of Whirlpools pay for executing his wife and approached the Lightning Daimyō to convince him to approve him to use the resources of Land of Lightning to launch an invasion to eradicate the Uzumaki Clan and Uzushiogakure. Although the Lightning Daimyō didn't agreed with the plan but he knew the Raikage wouldn't listen to reasons not after the Uzushiogakure have executed his wife. And refusing the Raikage during such an emotional turmoil means a definite civil war so he agreed. Since the Lightning Daimyō didn't wanted to bear all the expenses of the invasion alone, he informed the Raikage of the offer of the Water Daimyō regarding a joint invasion. The Raikage then went on to convince the other Kages on how Uzushiogakure is too powerful of a village to be allowed to continue, how the Fūinjutsu of the Uzumaki Clan is a direct threat to the balance of power between the Five Great Shinobi Villages as it could easily neutralise the power of Jinchūriki. How Uzushio must be destroyed because they were an asset to the Hidden Leaf. Because while Uzushio's army wasn't particularly powerful, they were sort of the world's authorities on Fūinjutsu (Sealing techniques). So in a world where the balance of power is largely determined by who has what Tailed Beasts, Konoha having a monopoly on such powerful sealing techniques was quite dangerous to other Villages. The Raikage then went on to convince some other villages too who had suffered loses because of the Fūinjutsu of the Uzumaki Clan and succeeded in banding together some of the world's nations who viewed Uzushiogakure as a village too powerful to be allowed to continue to exist.
The Uzumaki Clan learnt of this ploy of the Raikage three days before the day of the invasion and they hatched a plan to counter the invasion while preparing accordingly. A day before the day of the invasion Ashina sent his son who was capable of performing space-time ninjutsu to retrieve Mito Uzumaki. The plan was that: Mito Uzumaki would unleash the Nine-tails on the invading army while the 'Uzushio Ten Brave' along with Ashina and other elites will fight the five Jinchūrikis, Kage and leaders of the other villages. After the Uzumaki eliminates or sufficiently injuries the targeted individuals and Mito Uzumaki damages the invading army enough, they will turn it into a battle of attrition forcing the army to retreat due to the lack of proper leadership, supplies and whatnot. It was a sound plan and although Uzushio would have suffered massive losses but they would have survived the invasion.
*How did then Uzushiogakure got destroyed?
It was because of Danzō's paranoia. Danzō feared that Uzushio after witnessing the power of Nine-Tails wouldn't give it back to them even though they have already given Kushina to them. Then there was a chance that Mito could die or even got captured by their enemies in the battle due to her weakened state caused by her old age and illness. Thus Konohagakure could potentially lose their Jinchūriki. And in such a situation, the other villages will definitely deploy their own Jinchūrikis to battle against Konoha since Mito's death or capture means not only the loss of their Jinchūriki and the Nine-Tails but also the destruction of Uzumaki Clan and Uzushiogakure.
Konoha won't have anything to defend against the enemy's Jinchūrikis, neither their own Jinchūriki nor the renowned Uzumaki Fūinjutsu. Sending Mito to the battle means the same as sending Konoha potentially to it's deathbed. So he sent his Root forces and killed off the son of Ashina and the Uzumaki squad before they could reach Konohagakure. After killing Ashina's son and the Uzumaki squad, the Root forces then went off to complete their second objective: To abduct Uzumaki children and salvage whatever they could before the invading army arrives. Unfortunately, some of the spies of the invading forces managed to tail these Root ANBU and found out about the secret entrance to the island of the Land of Whirlpools. The invading army used this route for a surprise attack on Uzushiogakure and sent three of their Jinchūriki along with a squadron of suicide squads who wrecked the village from inside, the Jinchūrikis transformed and started killing indiscriminately while the suicide squads targeted the sensitive areas of Uzushio. Left with no choice due to the unexpected attack, Ashina was forced to divert back three of the Ten Brave along with a small amount of Uzushio forces to save the village. By the time they reached the village and subdued the three Jinchūrikis and killed the suicide squads, the village was left in a terrible state, one-fourths of their civilians and clan children had been killed and two-thirds of the village had been decimated due to the surprise attack.
Although the frontlines fighters managed to hold their own against the invading army but many managed to bypass them. Those who bypassed them, managed to reach Uzushio and freed the subdued Jinchūrikis before they could be permanently sealed. Once freed the Jinchūrikis again started wrecking the hidden village.
During the chaos, Ashina somehow managed to send some of the civilians and clan children away from the island even though he failed to save his village, land, army, clan and his very own life. He then used a Forbidden Fūinjutsu that destroyed the entire village of Uzushiogakure turning it into a rubble, burning their secret scrolls, jutsu, etc and killing everybody within Uzushio, friend and foe alike and taking his life in exchange. As a side effect it also erected a permanent barrier over the village which couldn't be bypassed by anybody not even by an Uzumaki unless approved to enter by the Ashina's soul which is cursed and trapped to dwell forever within this barrier never being able to pass on to the Pure Land or even to Limbo until they find peace and closure as a punishment for using this forbidden fūinjutsu technique that utilizes the power of the Shinigami. The barrier will remain erected over the island until his soul move on.
The Uzumaki could have saved their village, even if it would have left their village massively weakened, by turning the battle into a battle of attrition since they had technologically superior weapons and their sealing jutsus but the unexpected attack of Jinchūrikis from their rear ruined their entire formation of the battle plan, resulting in their defeat.
*How did the Jinchūrikis survived the Forbidden Fūinjutsu of Ashina?
They were already defeated by the time the Forbidden Fūinjutsu is used thus were kept out of the battle hence they didn't die like others.
*Why did the Raikage didn't managed to capture some Uzumaki to create the Uzumaki Clan in his village?
He remained unconscious throughout the rest of the invasion after being injured in battle while fighting one of the Ten Brave thus he didn't manage to issue the order of capturing some Uzumaki alive. And those whom he had given such an order were already dead by the time the invasion ended.
*How did the allied army managed to successfully navigate around the many whirlpools that surrounds the island and act as a natural defense for Uzushio?
The Hoshigakure and Land of the Sky Shinobi played the most vital role in this invasion. They used their flying abilities granted by the Star Chakra and Mist Chakra Techniques to scout the invading force around the Whirlpools safely into the island of Uzushiogakure.
(A/N: Not all of the Hoshi and Sky forces were capable of the flying ability.)
*How did the invading forces eluded the protective barrier erected by the Uzumaki Clan over their Village and entered Uzushio?
The Yuki Clan used their clan's ultimate technique the Demonic Mirroring Ice Crystals, which is said to have been passed down through the generations. The enemy forces used these mirrors to bypass the protective barrier erected by Uzumaki, entering Uzushio and wrecking havoc all over.
*Who were Uzushio Ten Brave?
The Uzushio Ten Braves, also known as the Ten Heroes of Uzushio is a group mostly composed of the Grand Seal Masters of the Uzumaki Clan and the most elites of the Arainami Clan and later on of Tatsuguchi Clan. The Uzushio Ten Braves became famous after the First Shinobi World War, when they neutralised and captured the Seven-Tails' Jinchūriki and ended the entire invading army of Taki and their allies all by themselves and were viewed by the other enemy nations as the most dangerous organization of Uzushiogakure. This group was also one of the reasons which incited the invasion of Uzushiogakure and was considered the centre pillar of the village alongside the Leader of Uzushiogakure.
*How did the Uzushio Ten Brave died?
The invading force first drew out the strongest members aka the Uzushio Ten Brave. When they were worn out sufficient enough from fighting the invading force, they sent their Jinchuriki's from the other side to wreck havoc inside Uzushiogakure, forcing Ashina to send some back in the village. Then the Third Raikage  with his partner Blue B, the Third Mizukage with the seven Swordsman of the Mist along with the remaining two Jinchūrikis and the leaders of other villages finished off the weaken Ten Brave. Still Uzushiogakure bravely fought and managed to kill twenty one thousands of them despite being only around SEVEN thousand in numbers. Eventually Ashina used his Forbidden Fūinjutsu and the island sealed itself off. Those unfortunate ones who were sealed within the island either died of starvation or the seal killed them off after completely sucking their chakra slowly over the next few months.
*How many of the allied forces survived after the end of the invasion?
Only around three thousands of the total allied invading force survived the battle by the end of the invasion.
*What happened to the survivors of Uzushiogakure?
The survivors of Uzushio's destruction went into hiding and spread throughout the world to seek refuge, only to be hunted by the Hunter-nins of the other villages who wanted their techniques for themselves.
*Why did Iwagakure didn't participated in the invasion despite being an enemy of Konohagakure?
Iwagakure was also invited to join this alliance and was told the benefits of gaining the Uzumaki Seals and how it will help their village in dealing with their Jinchūrikis and Bijūs. But Ōnoki stubbornly refused to join it and stood with his decision until the end despite being heavily criticized by the upper echelons of his village. And thanks to this stubbornness of Ōnoki, Iwagakure didn't suffered the same fate(massive casualties) as the other invading villages thus this decision not only increased his reputation within Iwagakure but also made him all the more infamous in the other hidden villages as 'Ōnoki the Fence-Sitter'.
*Why did Sunagakure didn't participated in the invasion despite being an enemy of Konohagakure?
Sunagakure didn't participated due to all of them believing Bunpuku's sealing jutsu more than sufficient enough to deal with the One-Tails and they considered this invasion a waste of resources and foolishness.
*Why did Amegakure didn't participated in the invasion despite being an enemy of Konohagakure?
Amegakure also didn't take part in this event cause instead they wanted to use the chaos of this event to attack Howling Wolf Village and annexed their territory into the Land of Rains. However, with the help of blue fire powder, the Wolf village easily repelled Amegakure.
*What did Konohagakure do during this invasion?
Since nobody arrived to pick Mito Uzumaki back to Uzushio, she herself went to Uzushiogakure but due to her declining health and old age, it was a slow journey. On her way to the shorelines, she was interrupted and ambushed many times by the allied shinobi stationed to make sure that Konoha don't interfere. By the time she arrived at the port, the allied forces did the smart thing and had destroyed all of the boats, ships, etc. Mito had no choice but to watch helplessly as her kin were slaughtered and killed by the Allied invading army. Hiruzen regrettably cannot send any of his forces for help since he had all his forces spread thin, the enemies were specifically vicious on this day so he couldn't spare a single ninja. In this way Sunagakure, Iwagakure and other enemies of Konoha contributed in the fall of Uzushiogakure. When the war ended, the Third Hokage tried to locate and gather the Uzushio survivors, but he didn't find any. The only silver lining is that the Uzumaki Clan managed to somehow protect most of their secrets, and the invaders didn't get anything meaningful out of the invasion, aside from common goods, pain and misery.
*Why no surviving Uzumaki sought shelter in Konohagakure?
Because the Uzumaki took the use of the secret passage known only to Konohagakure by invading forces as a sign of betrayal from Konohagakure, not to mention the attack from the rear became the main reason for their defeat, since they were expecting help thus have left behind a minimal force who could maintain the barrier to protect the village from inside but instead they gained more enemies in the form of three Jinchūrikis and suicide squads.
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There are many other factors that lead to the destruction of Uzushiogakure, some of them are:-
(i) During the era of Warring Clan States, the Uzumaki Clan used to have a policy known as Isolation Policy of Uzumaki Clan which played a key role in their destruction.
(ii) Their Fuinjutsu, which was in a class by itself.
(iii) The alliance with Konohagakure.
(iv) Danzō Shimura(Read it)
(v) Grand Seal Masters
(vi) Uzushio Ten Brave
(vii) Wealth and Technology
(viii) Their closeness with the Daimyōs and various other influential people.
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ISOLATION POLICY OF UZUMAKI CLAN
It was introduced in the Uzumaki clan during the Era of the Warring States, just after the Uzumaki decided to settle in the island that they named the 'Land of Whirlpools'. It was proposed by one of the then elders of the Uzumaki clan named Shisio Uzumaki and approved by the 35th leader of the Uzumaki clan Masahiro Uzumaki at that time.
During the Era of the Warring States Period, an Uzumaki whose ship was wrecked in a storm accidentally landed on the offshore of an island surrounded by Whirlpools. Somehow the Uzumaki managed to navigate around the Whirlpools and returned back to his clan. He then informed about this island to Masahiro Uzumaki who decided to settle on this island to protect his clan. Additionally, he also generated artificial Whirlpools all around the island to further secure the island and his clan from outside threats, thus creating a safe heaven for the Uzumaki Clan. The then elders of the Uzumaki clan named this artificially modified island, 'The Land of Whirlpools' based upon the numerous artificially created and natural Whirlpools protecting it.
The ISOLATION POLICY OF UZUMAKI CLAN states that only an Uzumaki can reside within the sacred 'Land of Whirlpools'. Later, it was changed a little bit when the families of wealthy and most trusted merchants who were seeking protection shown interest to settle down in the Land of Whirlpools. Hence they along with their families, colleagues and servants were allowed to settle down within the protected island. These migrated merchants were often used to be the frequent clients who sought out the services of Uzumaki clan during the Warring States Period. This decision greatly benefited the Land of Whirlpools making it a very wealthy nation. The wealth of Uzushiogakure also played a major role in its destruction because some of the nations were jealous of it and wanted it for themselves, so they attacked. This policy had helped sustain the Uzumaki Clan's continuous survival and prosperity even in the Era of Warring States Period. Thus Ashina Uzumaki hesitated and took too much time to terminate it.
Due to this hesitation of Ashina, the Uzumaki clan by proxy Uzushiogakure delayed the termination of Isolation policy for the next three years since the formation of Konohagakure. He forever regretted this hesitation/decision of his which made Uzushiogakure very heavily dependent upon Konohagakure for protection from other hidden villages. Because by the time he terminated this policy, it was already too late, most of the other prominent clans have already had joined other hidden villages for protection. Hence, Uzumaki clan never got the chance to invite and recruit many clans into their village to increase its own military strength. Uzushiogakure paid the ultimate price in the form of it's annihilation and Ashina Uzumaki until his last breath blamed himself for Uzushio's destruction. He died crying and blaming himself until he used the Forbidden Fūinjutsu to kill numerous invaders.
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FŪINJUTSU HAS FOUR CATEGORIES:-
(i) Sealing Tag Fūinjutsu:- Fūinjutsu through Sealing tags, Barrier tags, Explosive Tags, Elemental tags, Curse Tags, etc
(ii) Scroll Based Fūinjutsu:- Fūinjutsu through various kinds of scrolls and ink brush.
(iii) Chakra Based Fūinjutsu:- Fūinjutsu based on writing seals only using chakra like the Five Elements Seal or blood.
(iv) Hand Seals Based Fūinjutsu:- Fūinjutsu through hand seals.
But Uzumaki Clan created and added three more categories into it:-
(v) Buki-Fūinjutsu:- Fūinjutsu through weapons
(vi) Tattoo Fūinjutsu:- The most prized form of Fūinjutsu of the Uzumaki Clan like the Seal of the Grandmaster, Dōjutsu seal, Summoning seals on the wrist, storage seal on the hands, etc.
(vii) Object Fūinjutsu:- Binding powerful entities to objects like the Masks of the Uzumaki Temple.
*Juinjutsu and to some extent Jujutsu are sub-categories of Fūinjutsu as they are highly dependent on seals.
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CATEGORY OF SHINOBI IN FŪINJUTSU:-
(i) Shinobi Capable of performing Sealing Tag Fūinjutsu and some of the Scroll Based Fūinjutsu is termed as Fūinjutsu User.
(ii) Shinobi Capable of performing Sealing Tag Fūinjutsu and Scroll Based Fūinjutsu is called Fūinjutsu Master.
(iii) Shinobi Capable of performing Sealing Tag Fūinjutsu, Scroll Based Fūinjutsu, Chakra Based Fūinjutsu and Hand Seals Based Fūinjutsu is called Seal Master.
(iv) Shinobi Capable of performing all of the seven categories of Fūinjutsu is termed as Grand Seal Master. It is a rank exclusive only to the Uzumaki Clan since the last three forms are mostly known to them.
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The UZUMAKI FŪINJUTSU is RENOWNED because of:-
(i) their ability to repair themselves, i.e., they are Self-repairing Seals. They could repair themselves instantaneously if somehow damaged or is destroyed and requires very minimum chakra to do so and are very efficient.
(ii) high-power elemental fūinjutsu, i.e., they could bypass the effort needed to master and use the five elemental chakra as the fūinjutsu does everything to change the chakra nature and grant the user to use their chosen elements.
(iii) other sealing jutsu may takes time to show there effects but the Uzumaki Sealing takes no such time, it happens instantaneously at such a speed that they can catch even attentive opponents, such as a Sharingan user off guard.
(iv) miscellaneous seals which can mimic the effects of certain kekkei genkeis.
(v) their seals can be used over weapons and even skin very easily and aren't harmful to the receiver unlike most other sealing jutsus.
(vi) their seals are capable of sealing powerful beasts, summons and Bijūs without much drawbacks and very easily.
(vii) their seals are one of the best when it came to security and fortification, etc.
All of these qualities makes their sealing so much valuable and desirable. But what made their Fūinjutsu most UNIQUE is: No one else who was not an Uzumaki or don't know about their secrets could undo/remove their fūinjutsu. It was a ridiculous ability of the Uzumaki Clan. Even the simplest of their sealing jutsus cannot be undone by the other Seal Masters. It because even though the sealing jutsus remain simple, the sealing formula used by the Uzumaki Clan in their fūinjutsu is a complex web of formulas, those who doesn't know their secret simply could not understand it. Thus they cannot undo the Sealing Jutsu. It was like the hand writing of a child. Unless he said the words, you were not going to figure out what he wrote.
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FŪINJUTSU OF THE UZUMAKI CLAN
The Uzumaki clan was specialised in this particular branch of arts since the ancient times. They were in a class by himself in this branch with no equal. They were the only village which possess several grand seal masters of the highest calibre. They were capable of sealing anything be it a beast, summon, Bijū, dark entity, etc and utilised Fūinjutsu in every single branch of the other Ninja Arts. Their Kekkai Genkai Adamantine Chains which was also a form of Fūinjutsu. These things made their Fūinjutsu very dangerous and feared throughout the nations. But the real reasons the other nations started fearing them all the more after the First Shinobi World War, when Ashina Uzumaki accompanied by his fellow Grand Seal Masters fought and captured the Jinchūriki of the Seven-Tails of Takigakure(Hidden Waterfall Village) despite the said Jinchūriki going berserk and being in his fully transformed tailed beast state. Later, they also obliterated the advancing forces of Takigakure using different kind of seals and were forced to hand over the captured Jinchūriki of Seven-Tails back to Konohagakure by the First Hokage. It was at this point that the other nations stopped using their Bijū and lost their edge against Konohagakure in the war thus allowing Konohagakure to emerge victorious in the First Shinobi World War.
This events led to some of the world's nations in believing that Uzushiogakure had became too powerful to be allowed to continue. So the Raikage easily formed a temporary alliance and banded together to destroy Uzushiogakure and ultimately succeeded.
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THE ALLIANCE WITH KONOHAGAKURE
The other nations believed that with the help of Uzushiogakure, Konoha might become too powerful to be stopped. Since because of Uzushiogakure they had already lost the First Shinobi World War to Konoha. The other nations realised that as long as the Uzumaki clan exist they can't utilise their Jinchurikis against Konoha and thus can never get an edge over them. To other villages the Jinchūriki and their Bijū is a major power that could deter people from making false moves against them. But it won't stop Konoha from making ill moves against their villages, they could even launch an invasion and get away with it because in the presence of Uzumaki Clan, the war deterrent aka the power of the Bijū becomes irrelevant and useless. So they started waiting for a chance to destroy them and it came in the Second Shinobi World War. In time Uzushiogakure became over dependent on Konohagakure as such Danzo Shimura took advantage of it and manipulated Hiruzen into forcing Ashina to give them their powerful Sealing Jutsus in exchange for continuing the Alliance between their villages. Ashina reluctantly agreed and gave those seals to Konohagakure, making Konohagakure all the more powerful while their enemies all the more anxious.
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DANZŌ SHIMURA
There were probably three more reasons why Danzō played a part in their Destruction other than the one already explained:-
The alliance with Uzushiogakure must have been very vital to Konohagakure thus Uzushiogakure must have a great deal of political influence in the Land of Fire since they mostly took care of the security of Fire Daimyō and his councillors with their fūinjutsu. Hence they also used to have a lot of influence in choosing the Next Hokage. Remember the Jinchuriki of the Nine-Tails came from their clan. Danzō must have known that the Uzumaki Clan can cause some problems in his ascension to the throne of Konohagakure and they will never agree with his methods of turning the Jinchūriki of the Nine-Tails into an emotionless weapon. One other and the most important reason for Danzō was to ensure the absolute loyalty of the Nine-Tails' Jinchūriki to Konohagakure and Konohagakure alone. Because with the existence of Uzushiogakure, the loyalty of the Jinchūriki was divided between Konoha and Uzushio.
The last reason is that he must have thought that the survivors will take shelter in Konohagakure and as such the Uzumaki Clan will be reformed in his village and it will made them all the more stronger, not to mention, he might get to recruit some of the members of the Uzumaki Clan capable of subduing a Bijū in his Root forces. But alas the survivors never came to Konohagakure.
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WEALTH AND TECHNOLOGY OF UZUSHIOGAKURE
Due to the Change in the policy of the Uzumaki Clan during the Warring States Period, the wealth of the Land of Whirlpools significantly increased and as such many envied it and wanted it for themselves. After the migration of the Land of Neck, it increased all the more making Uzushiogakure an enticing target for some greedy Nations. Not to mention due to its unique location in the Central Elemental Sea, it made the Land of Whirlpools one of the most attractive hub for the shipping industry, trade and commerce. Due to these reasons, it made the Land of Water uneasy in signing a peace treaty that would further the Land of Whirlpools' stability and may increase their business all the more.
The technology of Uzushio was also considered unusually advanced when compared to other villages in that era, the village was mostly composed of several high-rise buildings made of stone which were fully covered in small seals, reminiscent of their clan's symbol: a spiral crest. They had already possessed Power Generators, Electricity, Television, Telephones, Wireless Radio, Transportation devices similar to Moon Transportation Device, Railways, Camera, Eye Scope, Telescope, Video Cassette Recorder, Satellite Dish, Computer, Volley Guns capable of firing Kunai, Chakra Transmission Communication Devices, flamethrower Cannons, Mechanical Drilling Arm, Drilling Machines, Retractable Cable Arm, a sort of Cranes, Metal Gauntlets, Lightning Barrel Launchers, Chakra Cannons, Flare Gun, Microscope, DNA Testing Machine, Wrist-Mounted Senbon Launcher, Blood Increasing Pill, Military Rations Pill, etc way ahead of other villages. Many of these technologies were also created by their village thanks to their usage of fūinjutsu in technologies.
The Uzumaki Clan also possessed a great deal of political influence over the Daimyōs of countries which lacked Hidden Shinobi Villages. These Daimyōs often hired and tasked them with fortifying their castle and fortress with their reinforcement and security seals, barriers, etc; almost every year. Since they were tasked with their security, the Daimyōs often listened to their suggestions and indulged their requests. These tasks generated large revenues increasing the Uzumaki Clan's already impressive wealth. They were also used to be hired by various influential, rich and paranoid people who also paid them handsomely for enhancing their security or for their services. It made many people look at the Uzumaki Clan with jealousy and envy.