The Kingdom of Pagan was the first Burmese kingdom to unify the regions that would later constitute modern-day Myanmar. Pagan's 250-year rule over the Irrawaddy valley and its periphery laid the foundation for the ascent of Burmese language and culture, the spread of Bamar ethnicity in Upper Myanmar, and the growth of Theravada Buddhism in Myanmar and in mainland Southeast Asia.^[22]

The kingdom grew out of a small 9th-century settlement at Pagan (present-day Bagan) by the Mranma/Burmans, who had recently entered the Irrawaddy valley from the Kingdom of Nanzhao. Over the next two hundred years, the small principality gradually grew to absorb its surrounding regions until the 1050s and 1060s when King Anawrahta founded the Pagan Empire, for the first time unifying under one polity the Irrawaddy valley and its periphery. By the late 12th century, Anawrahta's successors had extended their influence farther to the south into the upper Malay peninsula, to the east at least to the Salween river, in the farther north to below the current China border, and to the west, in northern Arakan and the Chin Hills.^[23] In the 12th and 13th centuries, Pagan, alongside the Khmer Empire, was one of two main empires in mainland Southeast Asia.^[24]

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 at all social strata. Pagan's rulers built over 10,000 Buddhist temples in the Bagan Archaeological Zone of which over 2000 remain. The wealthy donated tax-free land to religious authorities.^[25]

The kingdom went into decline in the mid-13th century as the continuous growth of tax-free religious wealth by the 1280s had severely affected the crown's ability to retain the loyalty of courtiers and military servicemen. This ushered in a vicious circle of internal disorders and external challenges by the Arakanese, Mons, Mongols and Shans. Repeated Mongol invasions (1277–1301) toppled the four-century-old kingdom in 1287. The collapse was followed by 250 years of political fragmentation that lasted well into the 16th century.^{[26 ]}The Pagan Kingdom was irreparably broken up into several small kingdoms. By the mid-14th century, the country had become organised along four major power centres: Upper Burma, Lower Burma, Shan States and Arakan. Many of the power centres were themselves made up of (often loosely held) minor kingdoms or princely states. This era was marked by a series of wars and switching alliances. Smaller kingdoms played a precarious game of paying allegiance to more powerful states, sometimes simultaneously.