Early civilisations in Myanmar included the Tibeto-Burman-speaking Pyu city-states in Upper Burma and the Mon kingdoms in Lower Burma. In the 9th century, the Bamar people entered the upper Irrawaddy valley and, following the establishment of the Pagan Kingdom in the 1050s, the Burmese language, culture and Theravada Buddhism slowly became dominant in the country. The Pagan Kingdom fell due to the Mongol invasions and several warring states emerged. In the 16th century, reunified by the Taungoo Dynasty, the country was for a brief period the largest empire in the history of Southeast Asia. The early 19th century Konbaung Dynasty ruled over an area that included modern Myanmar and briefly controlled Manipur and Assam as well. The British conquered Myanmar after three Anglo-Burmese Wars in the 19th century and the country became a British colony. Myanmar became an independent nation in 1948, initially as a democratic nation and then, following a coup d'état in 1962, a military dictatorship.
When did the dominant religion of Myanmar become something outside of the mainstream ?
in the 1050s, the Burmese language, culture and Theravada Buddhism slowly became dominant in the country.