During Pagan Dynasty, The Bamar people who are the majority ethnic group of Myanmar had just settled the country after Nanzhao Kingdom invasion who conquered the Pyu city states.
Being continued under competent leaders like Kyansittha but would eventually end with the First Mongol invasion of Burma.
This period also lead to the rising of Kingdom of Mrauk U who was founded by the Rakhein people after their migration to the Arakan Beginning in 900s and consolidating their power in the region in 1100s.
During this period of Burmese history the infamous Forty Years' War was fought between Kingdom of Ava and Hanthawaddy kingdom leading to rises of Burmese folk heroes like Minye Kyawswa, who was one of the commanders during the Forty Years' War.
But after Tabinshwehti assassniation he would be succeed by Bayinnaung who would expand Myanmar to its greatest extent in history and conquering the Ayutthaya Kingdom and Lan Xang as well as reintegrating the Shan states.
But shortly after Bayinnaung's death his accomplishments would be reversed by his son Nanda Bayin, who constantly fought with his vassals leading to the weakening of the First Toungoo empire.