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.

The UPA was formally disbanded in early September 1949, but some of its units continued operations until late 1956. Officially, the UPA's last military engagement occurred in October 1956, when remnants of the group fought on the Hungarian border region in support of that country's revolution. In March 2019, surviving UPA members were officially granted the status of veterans by the government of Ukraine.

The UPA has a mixed legacy, both in Ukraine and abroad. While commemorated by many Ukrainians as heroes of their nation, some Soviet Army veterans oppose their positive remembrance and only some UPA veterans have received official veteran status in March 2019, despite receiving other forms of commemoration along with monuments and memorials. Some UPA veterans have responded to Polish grievances over their past misdeeds by meeting with Polish veterans and apologizing. Despite controversies over the exhumation of UPA victims in Volhynia, Ukrainian and Polish historians have also collaborated on a multi-volume history of the two nations, including the fraught history during the UPA's era.

The UPA's command structure overlapped with that of the OUN-B (the more radical faction of the OUN after it split in 1940); local OUN and UPA leaders were frequently the same person. The OUN's military referents were the superiors of UPA unit commanders. The UPA was established in Volhynia and initially limited its activities to this region. Its first commander was the OUN military referent for Volhynia and Polesia, Vasyl Ivakhiv. In July, the UPA Supreme Command was organized with Dmytro Klyachkivsky at its head.

Organizationally, the UPA was divided into regions. the Western Operational Group operated in western Ukraine; the Southern Operational Group in the central-southern regions of Podolia and parts of Kyiv, Zhytomyr and Odesa oblasts; the Northern Operational Group in the northern regions of Volhynia, Rivne Oblast, and parts of Kyiv and Zhytomyr oblasts; in eastern Ukraine, the UPA fled north, as the Soviet Union had executed a number of the UPA's participants. The members of the Eastern Operational Group joined other UPA units in Dnipropetrovsk and Chernihiv oblasts.

In November 1943, the UPA adopted a new structure, creating a Main Military Headquarters and the General Staff. Roman Shukhevych headed the HQ, while Dmytro Hrytsai became chief of staff. The General Staff consisted of operations, intelligence, logistics, personnel, training, political education, and military inspectors departments. In addition to the three regions named above, there was also an attempt to create an Eastern Operational Group, including Kyiv and Zhytomyr oblasts, but the project never came to fruition. Similarly, the UPA-South region ceased to exist in the summer of 1944, but continued to appear in documents. Three military schools for low-level command staff were also established.^{[citation needed]}