Poles and Ukrainians have been rivals and enemies for centuries. After a relatively peaceful period in the 1920s and 1930s the world war initiated new horrors on an unprecedented scale in that geographical area - between Russia and Germany. I did not know that much about it until sometime in the mid 70s I read the most horrific book “Hell’s Mouth” a memoir by a Ukrainian count. Horrifying but a must read for anyone interested in the history of these troubled lands:
https://www.amazon.com/Hells-Mouth-Confessions-Count-Nepomuk/dp/0432104003
A possible source of the animosity may be religious - Catholic Poland and Orthodox Ukraine.
There was the 1918-1921 war where Poland saved Western Europe from the Communists. (Continued until 1923 in Lituania).