Bahman was in Ctesiphon, gathering an army on the order of his Shahanshah. Another commander, Andarzaghar already left with the first army, while Bahman was supposed to gather a second one, then meet at Walaja, where they would stop muslim advances on Hira. Bahman was impatient, as he was already delayed too much and feared he wouldn't get there quickly enough. He eventually gathered an army of about 10000 men from all of the surrounding towns garrisons. He knew his Shahanshah was supposed to arrive soon in Ctesiphon, and wanted to gift him a great victory. He set off to Walaja, along the two rivers, Euphrates and Tigris, while Andarzaghar went along the southern shore of Euphrates. Bahman set off a week after Andarzaghar did, which was later than he expected. Never would he thought it'll prove fatal.
Andarzaghar marched along the river with an army of around 15000. Along the way, he encountered remaining soldiers that survived the Battle of Chains and Battle of River, joining him and his purpose. In the towns, a lot of non-muslim Arabs expressed their desire to join the fight. When he arrived at Walaja and set up the camp, his army already numbered over 20000. He not only outnumbered the enemy himself, but with help from Bahman's army, the victory should be ensured. Should be.
Khalid's scouts and spies, who were mostly Arabs hostile to Persians reported gathering of two big armies. Apparently, they were yet to join together near a village called Walaja. Khalid couldn't take them on himself – their numbers were way too high. And, also according to the reports, Bahman Jaduya was supposed to command both of the armies. And although Khalid was a military genius, he lacked the confidence to stand against Bahman Jaduya, prestigious and renowned general everyone at least heard of once, while outnumbered and at a disadvantage. He had to destroy one of the armies before the second would arrive. So he advanced rapidly and quickly, arriving at Walaja and camping near few days before Bahman was expected to arrive. It was a great opportunity and Khalid was not about to let it go. He observed the Persian army very well. They were outnumbered a bit, but not by that much. And, Khalid already defeated armies that outnumbered him by far. He didn't think he won't be able to do it again. He already had a plan in mind.
Andarzaghar was alerted upon the arrival of Muslim army. He became nervous and anxious, as he counted with Bahman and his troops to aid him. But he didn't lose his head. He planned a strategy for the battle, wanting to stay on the defensive and fend off Arab attacks, until they'll wear off. Then he'll counter-attack and destroy the routing enemy. The plan relied on the fact that Arabs will go on the offensive and also whether Andarzaghar will choose the right time to counterattack. He was so invested in planning he didn't even see the Muslim cavalry moving behind them, behind the ridge.
Khalid, as he was preparing for tomorrow's battle, ordered his cavalry, under the cover of night to advance behind the enemy lines and attack on his signal. This strike right in the Persian back was supposed to completely disintegrate and crush Persian rear, should it be conducted in the right moment. And Khalid was positive he'll find that right moment. He shall.
Next morning, sun started shining, birds chirping and wildlife woke up. Andarzaghar had already deployed his army, as Khalid also did. Andarzaghar found it strange that the Arab army had very few cavalrymen. But he assumed they have them hidden somewhere, in the rear maybe, to serve as reinforcement or maybe a surprise attack on the advancing Persians. Never would he thought that he was right, partially, as Arab cavalry was in the rear – in his armies rear. Andarzaghar only made a formal speech, as the stories of Arab brutality and bloodshed they conducted already spread from the survivors of previous battles into the ears of the rest of the soldiers. They had their purpose already – to stop Arabs once and for all – to not let them through.
Horns roared and soldiers charged. Khalid ordered a general attack on Persian lines. And wasn't very successful. Not like he expected to be. Persians held them off pretty easily, while quickly replenishing those who died on the frontlines. Andarzaghar was there, with them, also fighting. And Khalid did the same. Arabs charged, were fend off, but charged again. After a third charge, Andarzaghar made a fatal mistake. He lost the patience and ordered a pursue. Right into Khalid's hands. Persians charged the Arabs, who in the meantime formed a line, and pursued them. Arabs, bit by bit, in the middle started to back off, while holding still on the sides. After a while, Arab line formed a shape of crescent. A symbol of Islam. And crescent is filled with full moon. The middle were Persians, while the outer lines were Arabs. War horn roared three times in a row. After a while, four thousand cavalry units appeared from behind the ridge, charging onto Persian exposed rear and making them the filling of a full moon.
Andarzaghar found out about the encirclement while it was too late. His soldiers were already trapped inside it with no escape. No hole in the Arab lines, no weak spot. He though he was done for. But refused to give it up now. He quickly made some orders to ensure proper defence against the cavalry units. That was his best bet. The Arab cavalry represented the weakest spot in the Arab line. If he didn't break through there, he wouldn't break through anywhere. He ordered his men to concentrate most of their pressure there, while fending off the Arab infantry. Being encircled isn't anything great – this tactic is used to defeat much larger armies. And there wasn't much you could do when encircled. Only fight and pray.
Bahman arrived at Walaja, only to see the Persians encircled by the Arabs, desperately trying to fight their way out. He noticed the concentration on the cavalry unit. He quickly called his commander of Aswaran, a heavily armoured cavalry, and ordered him to charge right into the Arab cavalry unit and to make space for the Persians to retreat. This mission was well-suited for heavy cavalry, as their armour allowed the to hold their ground for a prolonged period. He then watched them as they were accelerating themselves, preparing for the deadly charge.
Andarzaghar was in awe as he saw the Aswaran charge down the hill right into the Arab cavalry. They also, incidentally or not, hit the infantry as well. No matter, Andarzaghar now had the room to retreat, and he did just that. He ordered his army to retreat through the holes the Aswaran made by their charge. Thousands of survivors were flooding out, most quick enough, but some were cut down during their fleeing. They retreated, and so did the Arabs. Khalid knew he couldn't take on a new, fresh army, even if he had the numbers. If he didn't retreat now, Persians could group up and pursue him, with him having no chance of winning. And so, he retreated, leaving hundreds of his infantrymen and cavalrymen to their fate at the hands of Aswaran.
Andarzaghar barely retreated. Allthough most of his army survived the onslaught, the battle was clearly lost, even if he managed to hold the Arabs off. Ten thousand Persians laid dead on the battlefield, while only few thousand Arabs did. This battle wasn't worth it. It wasn't worth the price Persians paid. Arabs still had the strength to reinforce and continue on Hira, while Persians first had to regroup, reorganise and build a new army. Andarzaghar with Bahman decided to retreat back behind the Euphrates and discuss this matter with their Shahanshah, who should be arriving in Ctesiphon shortly.