Humayun`s war with Sher Shah Suri is an important chapter in the history of medieval India. There was a number of combat between the Afghan and the Mughal ruler where Humayun had to finally meet with defeat. While Humayun was busy in fighting against Bahadur Shah, the ruler of Gujarat, Sher Khan consolidated his position in Bihar. He had become the master of south Bihar, and was in possession of the strong fort of Chunar and most of the Afghan nobles had gathered under his banner. In Bengal, Mahmud Shah proved an incapable ruler. That gave further opportunity to Sher Khan to strengthen his power at the cost of Bengal. He attacked Bengal in 1526 A.D., besieged its capital Gaur and forced Mahmud Shah to pay thirteen lakh dinars. In 1537 A.D., he again attacked Bengal. Only then Humayun realised that it was necessary to subdue Sher Khan.
In July 1537 A.D. Humayun proceeded towards Bihar and first laid the siege of Chunargarh. Humayun could capture the fort after six months. In the meantime Sher Shah had captured Gaur and looted all its treasure which he kept safe at the Rohtasgarh fort. Humayun, thus, lost valuable time in the siege of Chunargarh. Humayun reached Varanasi and negotiated with Sher Khan for peace. It was agreed that the province of Bengal would be handed over to Sher Khan under the suzerainty of the Mughals and would pay ten lakh rupees annually while Bihar would be taken over by the Mughals. But Humayun broke off the negotiations with Sher Khan and proceeded towards Bengal. Sher Khan deputed his son Jalal Khan to delay the advance of Humayun. Jalal Khan successfully achieved his mission and returned to his father who had successfully finished his campaign in Bengal and returned to Bihar. Humayun therefore faced no difficulty in capturing Bengal. During these months, Sher Khan captured Kara, Bana, Sambhal etc. and laid siege of Chunargarh and Jaunpur. He blocked the way of return of Humayun to Agra.
Humayun took the Grand Trunk Road which passed through south Bihar and which 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 Shah also reached there. 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. It did rain and on 25th June he 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 entire army was destroyed. Humayun just saved his life by plunging himself into the river Ganges. The battle of Chausa was an important event in the history of medieval India.
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. While Sher Shah was consolidating his position in the east Humayun and his brothers wasted their time at Agra. The Mughals, of course, defeated Afghan army in Malwa which was sent by Sher Shah under his son Qutub Khan. But, they failed to take any effective measure against Sher Shah. However, Humayun finally moved towards the east and reached near Kannauj where Sher Shah had already encamped himself. This time too the two armies faced each other for more than a month and again the rains started. On 17 May 1540 A.D. when the Mughals were shifting themselves to a higher plain, Sher Shah attacked them. The
Mughals fought valiantly but were defeated. Humayun again fled away. This battle was the decisive battle between Humayun and Sher Shah. Humayun could reach Agra but had to flee as Sher Shah was pursuing him. Sher Shah captured Delhi and Agra and, thus, the Afghans snatched the throne of Delhi from the hands of the Mughals. Humayun first went to Lahore and finally left India to seek refuge at the court of Shah of Persia.