quality:correction, structural:summarization
Pagan Kingdom

Pagan gradually grew to absorb its surrounding states until the 1050s–1060s when Anawrahta founded the Pagan Kingdom, 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, Hinduism, and folk religion remained heavily entrenched. Pagan's rulers and wealthy built over 10,000 Buddhist temples in the Pagan capital zone alone. Repeated Mongol invasions in the late 13th century 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 sizeable powers, Ava Kingdom and Hanthawaddy Kingdom, emerged. In the west, a politically fragmented Arakan was under competing influences of its stronger neighbours until the Kingdom of Mrauk U unified the Arakan coastline for the first time in 1437. The kingdom was a protectorate of the Bengal Sultanate at different time periods.

In the 14th and 15th centuries, Ava fought wars of unification but could never quite reassemble the lost empire. Having held off Ava, the Mon -speaking 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 and ruled Upper Myanmar until 1555.

Like the Pagan Empire, Ava, Hanthawaddy and the Shan states were all multi-ethnic polities. Despite the wars, cultural synchronisation continued. This period is considered a golden age for Burmese culture. Burmese literature "grew more confident, popular, and stylistically diverse", and the second generation of Burmese law codes as well as the earliest pan-Burma chronicles emerged. Hanthawaddy monarchs introduced religious reforms that later spread to the rest of the country.