Alp Arslan accompanied his uncle Tughril on campaigns in the south against the Fatimids while his father Chaghri remained in Khorasan. Upon Alp Arslan's return to Khorasan, he began his work in administration at his father's suggestion. While there, his father introduced him to Nizam al-Mulk, one of the most eminent statesmen in early Muslim history and Alp Arslan's future vizier.[6]
After the death of his father, Alp Arslan succeeded him as governor of Khorasan in 1059. His uncle Tughril died in 1063 and had designated his successor as Suleiman, Arslan's infant brother. Arslan and his uncle Kutalmish both contested this succession which was resolved at the battle of Damghan in 1063. Arslan defeated Kutalmish for the throne and succeeded on 27 April 1064 as sultan of the Seljuk Empire, thus becoming sole monarch of Persia from the river Oxus to the Tigris. In 1064 he led a campaign in Georgia during which he captured the regions between Tbilisi and the Çoruh river, Akhalkalaki and Alaverdi.[7] Bagrat IV submitted to paying jizya to the Seljuks but the Georgians broke the agreement in 1065.[8] Alp Arslan invaded Georgia again in 1068, he captured Tbilisi after a short battle and obtained the submission of Bagrat IV, however the Georgians freed themselves from Seljuk rule around 1073–1074.[8][9]
In consolidating his empire and subduing contending factions, Arslan was ably assisted by Nizam al-Mulk, and the two are credited with helping to stabilize the empire after the death of Tughril. With peace and security established in his dominions, Arslan convoked an assembly of the states and in 1066, he declared his son Malik Shah I his heir and successor.[10] With the hope of capturing Caesarea Mazaca, the capital of Cappadocia, he placed himself at the head of the Turkoman[11] cavalry, crossed the Euphrates, and entered and invaded the city. Along with Nizam al-Mulk, he then marched into Armenia and Georgia, which he conquered in 1064.[12] After a siege of 25 days, the Seljuks captured Ani, the capital city of Armenia.[13] An account of the sack and massacres in Ani is given by the historian Sibt ibn al-Jawzi, who quotes an eyewitness saying:
Putting the Persian sword to work, they spared no one... One could see there the grief and calamity of every age of human kind. For children were ravished from the embraces of their mothers and mercilessly hurled against rocks, while the mothers drenched them with tears and blood... The city became filled from one end to the other with bodies of the slain and [the bodies of the slain] became a road. [...] The army entered the city, massacred its inhabitants, pillaged and burned it, leaving it in ruins and taking prisoner all those who remained alive...The dead bodies were so many that they blocked the streets; one could not go anywhere without stepping over them. And the number of prisoners was not less than 50,000 souls. I was determined to enter city and see the destruction with my own eyes. I tried to find a street in which I would not have to walk over the corpses; but that was impossible.[14]