In spring 1943, the OUN-B's UPA launched a campaign of murder and expulsion against the Polish population of Volhynia, and in early 1944 against the Poles in Eastern Galicia.
This was done as a preemptive strike, in expectation of another Polish-Ukrainian conflict over the disputed territories, which were internationally recognized as part of Poland in 1923.

The Polish government in exile wanted to restore eastern Polish borders beyond the Curzon Line, an aim that was also supported by promises from the Western Allies.
The Ukrainian majority in Eastern Galicia, which was promised autonomy, initially considered the Polish administration that followed the Peace of Riga and the Polish-Soviet War to be illegitimate, but after 1923 most Ukrainians grudgingly accepted Polish rule as a fact they could not change and focused on increasing their autonomy.
Ukrainians in general were opposed to the Polish regime and all Ukrainian political movements had independence from Poland as a goal.
 Ukrainians took two approaches towards independence.
 The mainstream parties avoided violence and sought the goal of independence through gradual legal means focused on preserving Ukrainians' limited rights,  while the OUN sought to use terror and violence in opposition to the Polish government.
The OUN regarded Galicia and Volhynia as ethnic Ukrainian territory that should be included in a future restored Ukrainian republic.

It is estimated that up to 100,000 Poles were killed by the Ukrainian nationalists during the conflict and another 300,000 made refugees as a result of the ethnic cleansing.
According to Timothy Snyder, 40,000-60,000 Polish civilians were killed by the UPA in Volhynia in 1943, and some 25,000 in Eastern Galicia.
Conversely, killings of Ukrainians by Poles resulted in between 10,000 and 12,000 deaths in Volhynia, Eastern Galicia and present-day Polish territory.
University of Alberta historian Per Rudling has stated that Shukhevych commanded the UPA during the summer of 1943, when tens of thousands of Poles were massacred.
 However, the initiator of these massacres was Dmytro Klyachkivsky.
They reached their height in July 1943, while Shukhevych did not assume command of the OUN until August 25 of that year and command over the UPA until November 1943.
In spring 1943, the OUN-B's UPA launched a campaign of murder and expulsion against the Polish population of Volhynia, and in early 1944 against the Poles in Eastern Galicia. This was done as a preemptive strike, in expectation of another Polish-Ukrainian conflict over the disputed territories, which were internationally recognized as part of Poland in 1923.
The Polish government in exile wanted to restore eastern Polish borders beyond the Curzon Line, an aim that was also supported by promises from the Western Allies. The Ukrainian majority in Eastern Galicia, which was promised autonomy, initially considered the Polish administration that followed the Peace of Riga and the Polish-Soviet War to be illegitimate, but after 1923 most Ukrainians grudgingly accepted Polish rule as a fact they could not change and focused on increasing their autonomy. Ukrainians in general were opposed to the Polish regime and all Ukrainian political movements had independence from Poland as a goal.  Ukrainians took two approaches towards independence.  The mainstream parties avoided violence and sought the goal of independence through gradual legal means focused on preserving Ukrainians' limited rights,  while the OUN sought to use terror and violence in opposition to the Polish government. The OUN regarded Galicia and Volhynia as ethnic Ukrainian territory that should be included in a future restored Ukrainian republic.
It is estimated that up to 100,000 Poles were killed by the Ukrainian nationalists during the conflict and another 300,000 made refugees as a result of the ethnic cleansing. According to Timothy Snyder, 40,000-60,000 Polish civilians were killed by the UPA in Volhynia in 1943, and some 25,000 in Eastern Galicia. Conversely, killings of Ukrainians by Poles resulted in between 10,000 and 12,000 deaths in Volhynia, Eastern Galicia and present-day Polish territory. University of Alberta historian Per Rudling has stated that Shukhevych commanded the UPA during the summer of 1943, when tens of thousands of Poles were massacred.  However, the initiator of these massacres was Dmytro Klyachkivsky. They reached their height in July 1943, while Shukhevych did not assume command of the OUN until August 25 of that year and command over the UPA until November 1943.
Roman Shukhevych