In 1228, the Ahoms, a Buddhist tai tribe from north Thailand entered Assam and established a kingdom which came to be called Assam. Ahom power started weakening in the 17th century. Then in 1792, Burma invaded Assam. The Ahoms appealed to the British for help. The British defeated Burma in several battles. With the Treaty of Yandaboo in 1826, the Burmese vacated Assam, leaving its possession to the British. The conquered territory was placed under the administration of an agent to the Governor general. In 1874 a separate province of Assam under a chief commissioner was created with Shillong its capital. On the partition of Bengal in 1905, Assam was united to the eastern districts of Bengal. In 1921 a Governorship was created. In 1948 the North East Frontier Agency was separated from Assam, for security reasons. Later, states like Mizoram, Meghalaya, Nagaland, Tripura and Arunachal Pradesh were made from Assam.