The Ukrainian Insurgent Army (Ukrainian: Українська повстанська армія, УПА, romanized: Ukrainska Povstanska Armiia, abbreviated UPA) was a Ukrainian nationalist partisan formation founded by the Organisation of Ukrainian Nationalists (OUN) on 14 October 1942. The UPA launched guerrilla warfare against Nazi Germany, the Soviet Union, and both the Polish Underground State and Polish Communists. The UPA carried out massacres of Poles in Volhynia and Eastern Galicia, which are recognized by Poland as a genocide. The goal of the OUN was to establish an independent Ukrainian state. This goal, according to the OUN founding declaration, "was to be achieved by a national revolution led by a dictatorship" that would drive out occupying powers and then establish a "government representing all regions and social groups"; OUN accepted violence as a political tool against enemies of their cause. In order to achieve this goal, a number of partisan units were formed, merged into a single structure in the form of the UPA, which was created on 14 October 1942. From February 1943, the organization fought against the Germans in Volhynia and Polesia. At the same time, its forces fought against the Polish resistance, during which the UPA carried out massacres of Poles in Volhynia and Eastern Galicia, resulting in the deaths of up to 100,000 Polish civilians. In 1944, as the German army was retreating, the UPA continued its war against them by attacking its rear and seizing its equipment, but at the end of July 1944 the UPA formed a united front with Nazi Germany, ceasing attacks on the withdrawing Wehrmacht and defending against the Soviets in exchange for military aid. Soviet NKVD units fought against the UPA, which engaged in armed resistance against Soviets until 1949. On the territory of Communist Poland, the UPA tried to prevent the forced deportation of Ukrainians from western Galicia to the Soviet Union until 1947.
The UPA was a decentralized movement widespread throughout Ukraine, divided into three operational regions; each region followed a somewhat different agenda, given the circumstances of a constantly moving front line and a double threat from both Soviet and Nazi forces. Not all UPA soldiers were members of the OUN or shared OUN's ideology.