Your history is fuzzy...that was the SOVIET UNION, i.e. RUSSIA, that backstabbed Poland in 1939.
I grew up in a Polish neighborhood in Chicago went to a Polish Church with many veterans of the attack in 1939. They had their own Polish Veteran’s Post - every Polish veteran who fought in 1939 against both the Nazis and Russians despised the Ukrainians.
It was so bad that across the street from our Church was a Ukrainian Catholic Church - The Polish Veterans would cross the street rather than walk in from of the Ukrainian Church.