The massacres of Poles in Volhynia and Eastern Galicia (; ) were carried out in German-occupied Poland by the Ukrainian Insurgent Army (UPA) with the support of parts of the local Ukrainian population against the Polish minority in Volhynia, Eastern Galicia, parts of Polesia and Lublin region from 1943 to 1945. The ruling Germans also actively encouraged both Ukrainians and Poles to kill each other.The peak of the massacres took place in July and August 1943. The massacres were exceptionally brutal and affected primarily women and children. The UPA's actions resulted in up to 100,000 deaths. Other victims of the massacres included several hundred Armenians, Jews, Russians, Czechs, Georgians, and Ukrainians who were part of Polish families or opposed the UPA and sabotaged the massacres by hiding Polish escapees.
The ethnic cleansing was a Ukrainian attempt to prevent the post-war Polish state from asserting its sovereignty over Ukrainian-majority areas that had been part of the pre-war Polish state. The decision to force the Polish population to leave the areas considered by the Banderite faction of the Organization of Ukrainian Nationalists (OUN-B) to be Ukrainian took place at a meeting of military referents in the autumn of 1942, and it was planned to liquidate the leaders of the Polish community and those who would resist. Local UPA commanders in Volhynia, joining the armed uprising against the Germans, began attacking the Polish population, carrying out massacres in many villages. Encountering resistance, UPA commander in Volhynia Dmytro Klyachkivsky "Klym Savur" issued an order in June 1943 for the "general physical liquidation of the entire Polish population". The largest wave of attacks took place in July and August 1943, the assaults in Volhynia continued until the spring of 1944, when the Red Army arrived in Volhynia and the Polish underground, which had hitherto organised self-defences, formed the 27th AK Infantry Division. Approximately 50,000–60,000 Poles died as a result of the genocide in Volhynia, while up to 2,000–3,000 Ukrainians died as a result of Polish retaliatory actions.
At the 3rd OUN Congress in August 1943, Mykola Lebed criticised the Ukrainian Insurgent Army's actions in Volhynia as "bandit". The majority of delegates, however, opposed his assessment and the congress decided to carry the anti-Polish action into Galicia. However, it took a different course; by the end of 1943, it was limited to killing the leaders of the Polish community and exhorting Poles to flee to the west under the threat of looming genocide. In March 1944, the UPA command, headed by Roman Shuchevych, issued an order to drive Poles out of Eastern Galicia, first by warning and then by raiding villages, murdering men and burning buildings. A similar order was issued by the UPA commander in Eastern Galicia, Vasyl Sydor "Shelest". This order was often not obeyed and entire villages were slaughtered. In Eastern Galicia between 1943 and 1946, the OUN-B and the UPA killed 20,000–25,000 Poles. 1,000–2,000 Ukrainians were killed by the Polish underground.
Some Ukrainian religious authorities, institutions and leaders protested against the slayings of Polish civilians but without achieving much. In 2008, the Polish Parliament adopted a resolution defining the UPA's crimes against Poles as "crimes bearing the hallmarks of genocide". In 2013, it passed a resolution that it was "ethnic cleansing with the hallmarks of genocide". On 22 July 2016, the Sejm established 11 July as National Day of Remembrance for the Victims of Genocide committed by Ukrainian nationalists against the citizens of the Second Polish Republic. This classification is disputed by Ukraine and some non-Polish historians, who use the term "ethnic cleansing".
Background
Interwar period in Second Polish Republic
Recreated Polish state covered large territories inhabited by Ukrainians, while the Ukrainian movement failed to achieve independence. According to the Polish census of 1931, in Eastern Galicia, the Ukrainian language was spoken by 52% of the inhabitants, Polish by 40% and Yiddish by 7%, In Wołyn (Volhynia), the Ukrainian language was spoken by 68% of the inhabitants, Polish by 17%, Yiddish by 10%, German by 2%, Czech by 2% and Russian by 1%.