> USSR took back territories that Poland took from USSR in the 1919-1921 Polish-Soviet war
Essentially, Poland was occupying these territories between 1921 and 1939. Also, Poland had a dictatorship (quite typical in Europe at that time) and they were highly aggressive with respect to their neighbors: Polish-Ukrainian War (1918–19), Polish-Lithuanian War (1920, culminating in Żeligowski's Mutiny), Polish-Czechoslovak border conflicts (beginning in 1918).
Essentially, Poland was occupying these territories between 1921 and 1939. Also, Poland had a dictatorship (quite typical in Europe at that time) and they were highly aggressive with respect to their neighbors: Polish-Ukrainian War (1918–19), Polish-Lithuanian War (1920, culminating in Żeligowski's Mutiny), Polish-Czechoslovak border conflicts (beginning in 1918).