The rugged terrain of Myanmar's upland border areas has produced a number of ethnic minority groups that have not been absorbed into the state. They constitute more than 30 percent of Myanmar's population and are divided between five main highland ethnicities — Karen, Shan, Karenni, Chin and Kachin — and a number of smaller groups, including the Wa and Pa-Oh. The highland's rough terrain historically has shielded highland power centers from lowland domination and prevented the lowland from absorbing them into the population. Today, the highlands afford cover for a number of ethnic insurgent groups, which are supported by smuggling operations and foreign backing.
The territory of Myanmar has only been unified under a single power during three periods of its history: the Pagan Empire (849-1297), the Taungoo Dynasty (1486-1752) and the Konbaung Dynasty (1752-1885). None of these unifications resembled the modern conception of a state. Instead, the highland areas were governed lightly and operated more as buffer zones owing allegiance to the lowland Irrawaddy and to other powers. These Irrawaddy states were also inherently unstable, tending toward chaos and eventual fragmentation under pressure from the uplands. Pagan dissolved through Mongol and Shan invasions, Taungoo because of pressure from India's upland areas in Manipur and Thailand's Chiang Mai, and Konbaung because of the British invasion from the western uplands and a simultaneous sea assault. Myanmar's geopolitics have always made it difficult to govern the highlands, which are key to the governance of the lowlands.
British rule left Myanmar with a legacy of essentially artificial borders. The British formalized the borders of what had been until then a loosely organized territory. The British colonial government also aggravated ethnic divisions in the country by centralizing governance into a single source of power, ending the loose, at-will relations of the pre-British period. British administrations also relied on a "divide and rule" system that favored ethnic minority groups, which it called the "martial races" and used as the backbone of the British military in Myanmar to balance against the majority Bamar.