Imperial BurmaMain articles: Pagan Kingdom, Toungoo Dynasty, and Konbaung Dynasty See also: Ava Kingdom, Hanthawaddy Kingdom, Mrauk U Kingdom, and Shan states
Pagan gradually grew to absorb its surrounding states until the 1050s–1060s when Anawrahta founded the 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 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. Many splendid temples of Mrauk U were built during this period.
Political unification returned in the mid-16th century, due to the efforts of one tiny Toungoo (Taungoo), a former vassal state of Ava. Toungoo's young, ambitious king Tabinshwehti defeated the more powerful Hanthawaddy in 1541. His successor Bayinnaung went on to conquer a vast swath of mainland Southeast Asia including the Shan states, Lan Na, Manipur, the Chinese Shan states, Siam, Lan Xang and southern Arakan. However, the largest empire in the history of Southeast Asia unravelled soon after Bayinnaung's death in 1581, completely collapsing by 1599. Siam seized Tenasserim and Lan Na, and Portuguese mercenaries established Portuguese rule at Syriam (Thanlyin).
The dynasty regrouped and defeated the Portuguese in 1613 and Siam in 1614. It restored a smaller, more manageable kingdom, encompassing Lower Burma, Upper Burma, Shan states, Lan Na and upper Tenasserim. The Restored Toungoo kings created a legal and political framework whose basic features would continue well into the 19th century. The crown completely replaced the hereditary chieftainships with appointed governorships in the entire Irrawaddy valley, and greatly reduced the hereditary rights of Shan chiefs. Its trade and secular administrative reforms built a prosperous economy for more than 80 years. From the 1720s onward, the kingdom was beset with repeated Manipuri raids into Upper Burma, and a nagging rebellion in Lan Na. In 1740, the Mon of Lower Burma founded the Restored Hanthawaddy Kingdom. Hanthawaddy forces sacked Ava in 1752, ending the 266-year-old Toungoo Dynasty.
After the fall of Ava, one resistance group, Alaungpaya's Konbaung Dynasty defeated Restored Hanthawaddy, and by 1759, had reunited all of Burma (and Manipur), and driven out the French and the British who had provided arms to Hanthawaddy. By 1770, Alaungpaya's heirs had subdued much of Laos (1765), defeated Siam (1767), and defeated four invasions by China (1765–1769). With Burma preoccupied by the Chinese threat, Siam recovered its territories by 1770, and went on to capture Lan Na by 1776. Burma and Siam went to war until 1855, but all resulted in a stalemate, exchanging Tenasserim (to Burma) and Lan Na (to Siam). Faced with a powerful China and a resurgent Siam in the east, King Bodawpaya turned west, acquiring Arakan (1785), Manipur (1814) and Assam (1817). It was the second largest empire in Burmese history but also one with a long ill-defined border with British India.
The breadth of this empire was short lived. Burma lost Arakan, Manipur, Assam and Tenasserim to the British in the First Anglo-Burmese War (1824–1826). In 1852, the British easily seized Lower Burma in the Second Anglo-Burmese War. King Mindon tried to modernize the kingdom, and in 1875 narrowly avoided annexation by ceding the Karenni States. The British, alarmed by the consolidation of French Indo-China, annexed the remainder of the country in the Third Anglo-Burmese War in 1885.
Konbaung kings extended Restored Toungoo's administrative reforms, and achieved unprecedented levels of internal control and external expansion. For the first time in history, the Burmese language and culture came to predominate the entire Irrawaddy valley. The evolution and growth of Burmese literature and theater continued, aided by an extremely high adult male literacy rate for the era (half of all males and 5% of females). Nonetheless, the extent and pace of reforms were uneven and ultimately proved insufficient to stem the advance of British colonialism.