Battle of Chausa took place on 26th of June 1539 A.D. and was fought between the Afghan ruler Sher Khan and the Mughal Emperor Humayun. After conquering Bengal Humayun was heading towards Agra
> but Sher Khan had blocked the way of return of Humayun to Agra. Humayun took the Grand Trunk Road
which passed through south Bihar and was under complete control of Sher Khan. Humayun crossed the river Ganges and reached Chausa, a place at the boundary between Bihar
and Uttar Pradesh
. Sher Khan also reached there with his troops.
The two armies remained there facing each other for three months (April to June 1539 A.D.). Negotiations of peace were carried on but nothing came out of them. Sher Khan delayed the battle deliberately. He waited for the rains which could create problem for the Mughal army which was camping in between the rivers Ganges and Karmanasa. That actually happened when the rains started. On 25 June Sher Khan gave the impression to the Mughals that he was proceeding to subdue one of the tribal chiefs in Bihar. But, he returned and attacked the Mughals at night from three sides. The Mughals were completely surprised and the army was destroyed. Humayun just saved his life by plunging himself into the river Ganges.
Sher Khan declared himself the Sultan and assumed the title of Sher Shah after this battle. He captured Bengal well and then returned to Kannauj.