Pagan Empire, the first ever unification of the Irrawaddy valley and its periphery. In the 12th and 13th centuries, the Pagan Empire and the Khmer Empire were two main powers in mainland Southeast Asia. The Burmese language and culture gradually became dominant in the upper Irrawaddy valley, eclipsing the Pyu, Mon and Pali norms by the late 12th century. Theravada Buddhism slowly began to spread to the village level although Tantric, Mahayana, Brahmanic, and animist practices remained heavily entrenched. Pagan's rulers and wealthy built over 10,000 Buddhist temples in the Pagan capital zone alone. Repeated Mongol invasions (1277–1301) toppled the four-century-old kingdom in 1287.
Pagan's collapse was followed by 250 years of political fragmentation that lasted well into the 16th century. Like the Burmans four centuries earlier, Shan migrants who arrived with the Mongol invasions stayed behind. Several competing Shan states came to dominate the entire northwestern to eastern arc surrounding the Irrawaddy valley. The valley too was beset with petty states until the late 14th century when two sizable powers, Ava Kingdom and Hanthawaddy Kingdom, emerged. In the west, a politically fragmented Arakan was under competing influences of its stronger neighbors until the Kingdom of Mrauk U unified the Arakan coastline for the first time in 1437.
Early on, Ava fought wars of unification (1385–1424) but could never quite reassemble the lost empire. Having held off Ava, Hanthawaddy entered its golden age, and Arakan went on to become a power in its own right for the next 350 years. In contrast, constant warfare left Ava greatly weakened, and it slowly disintegrated from 1481 onward. In 1527, the Confederation of Shan States conquered Ava itself, and ruled Upper Burma until 1555.
Like the Pagan Empire, Ava, Hanthawaddy and the Shan states were all multi-ethnic polities. Despite the wars, cultural synchronization continued. This period is considered a
