Organization of Ukrainian Nationalists
The Ukrainian Insurgent Army (, "Ukrayins'ka povstans'ka armiya", UPA) was a Ukrainian nationalist paramilitary and later partisan formation. During World War II, it was engaged in guerrilla warfare against the Soviet Union, the Polish Underground State, Kingdom of Romania, and Communist Poland. It was established by the Organization of Ukrainian Nationalists. The insurgent army arose out of separate militant formations of the Organization of Ukrainian Nationalists—Bandera faction (the OUN-B), other militant national-patriotic formations, some former defectors of the Ukrainian Auxiliary Police, mobilization of local populations and others. The political leadership of the army belonged to the Organization of Ukrainian Nationalists—Bandera. It was the primary perpetrator of the ethnic cleansing of Poles in Volhynia and Eastern Galicia.