The two-day long battle of Bubaigawara (27-28 June 1333) saw the defenders of Kamakura miss an opportunity to defeat the pro-Imperial army of Nitta Yoshisada, thus ensuring that the fighting would move to the Shogunate's capital at Kamakura (Genko War).
Nitta Yoshisada had secretly joined the Imperial cause during the siege of Chihaya, where he had opened communications with the Imperial side and received an Imperial mandate to fight against the Shogunate. He feigned illness, returned home and began to raise an army.
His revolt was forced into the open after he captured and executed a tax collector sent from Kamakura, the capital of the Shogunate. On 20 June 1333 he raised his banner and began a march south towards Kamakura, gathering support as he went. The Shogunate responded by sending an army north to intercept him at the Iruma River, but this army was unable to prevent him from crossing the river. The first battle was fought at Kotesashi, a few miles south of the river, and was an inconclusive clash (battle of Kotesashi, 23 June 1333). The two sides separated, but the fighting was resumed on the following day. Once again Yoshisada attacked the Bakufu army, but this time his efforts resulted in victory (battle of Kumegawa, 24 June 1333).
The defeated Bakufu force pulled back to the Tama River, while messengers took news of the defeat back to Kamakura. Yoshisada paused at the Kume River to allow his men time to rest after two battles in two days, and this gave the Bakufu time to rush reinforcements north. Hojo Takatoki, the head of the Hojo clan and effectively the leader of the Shogunate, appointed his younger brother to command this army, which according to the Taiheiki was 100,000 strong. The original force was reported as being 60,000 strong, so the combined army would thus be somewhere under 160,000 strong, depending on the number of casualties suffered in the earlier battles.
The Taiheiki isn't a particularly reliable guide to the size of armies, combining a tendency to exaggeration with some inconsistencies. At its peak Yoshisada's army was said to have been 200,000 strong. His casualties are said to have been low at the first two battles, but after the first day of the battle of Bubaigawara his army was apparently only 100,000 strong. The events of the first two battles of the campaign suggest that Yoshisada did outnumber the first Bakufu force sent against him, but that after the arrival of reinforcements the Bakufu had the advantage.
Yoshisada finally decided to advance on the night of the 14th day (26-27 June 1333). The battle began late in the night (still the 14th day in the Japanese calendar, early on 27 June in the western calendar). Yoshisada's men let out a battle cry, but it was the reinforced Bakufu army that attacked first. They began by sending 3,000 archers forward to slow the rebel advance, before the rest of the army attacked. Yoshisada fought back, and his men launched seven or eight attacks, but they were outnumbered and were eventually forced to retreat north towards Horigane. This first part of the battle took place at Bubaigawara, modern Fuchu on the north bank of the Tama River (western Tokyo).
Although the forces of the Bakufu had won a significant victory, their morale was still poor and instead of pursuing Yoshisada's retreating men they held their position at Bubaigawara. They thus missed a chance to destroy Yoshisada's troops while they were still demoralised.
Late on the 15th day (27 June) reinforcements led by Miura Yoshikatsu reached Yoshisada's army. Yoshikatsu encouraged Yoshisada to attack on the following day, claiming that the Bakufu's men were overconfident and that his own fresh force would turn the tide.
On the morning of the 16th day (28 June) Miura Yoshikatsu led his men towards the Bakufu army, with their banners furled. The Bakufu men weren't expecting to be attacked, and were still in their camps as he advanced. They weren't sure who the newcomer was, with some worrying that they were enemies while most assumed that they were reinforcements.
This allowed Miura Yoshikatsu to get into a dangerous position. He was followed by Yoshisada with the main part of the army. This was split into three divisions, each of which advanced towards the Bakufu force from a different direction. When they got close they raised a battle cry. The Bakufu forces attempted to get to their horses, but as they were being attacked from the front by Yoshisada they were hit in the rear by Miura Yoshikatsu. This caused chaos in the Bakufu force which broke and fled south across the river and back towards Kamakura.
The second day of the battle is sometimes associated with Sekido, ten miles to the south-east of Bubaigawara, and on the opposite side of the river. This was where Hojo Takatoki's brother was nearly caught by the pursuing rebels. Only a dramatic last stand in which over 300 of his defenders were killed gave him the chance to escape to relative safety.
The news of this defeat reached Kamakura just before the news of the fall of the Rokuhara, their headquarters in the Imperial capital of Kyoto. This news also reached Yoshisada's army, greatly encouraging them as the prepared for the final stage in their campaign, the siege of Kamakura.