The Karen are an internally diverse group of ethnic minorities who live primarily in southern and southeastern Burma. They are the second-largest non-Burman ethnic group in Myanmar comprising some 6% of the population, and are mostly Christian. During WWII, roughly 28% of Karen served in the Burmese army, which by British policy deliberately excluded ethnic Burmans. During the war, the Karen continued to support the British even as Burmans, led by Aung San, sided with the Japanese.
Representatives for the Karen attended the 1947 Panglong Conference as observers but did not formally participate in the negotiations. Beginning in 1949, a rebellion broke out among the Karen people and was quickly followed by the wholesale defection of Karen units in the newly-formed Burmese army. The Karen National Union (KNU) has been at war with the central Burmese government since this time, making this rebellion the longest in contemporary world history. They seek greater autonomy, political rights (including the right to bear arms), and social autonomy in the realms of religion, culture, education and language.
Sources
David Steinberg, Burma/Myanmar: What Everyone Needs to Know (Oxford: Oxford University Press, 2010).
Martin Smith, Burma: Insurgency and the Politics of Ethnicity (New York: Zed Books, 1999).