> 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).
It had authoritarianism (with many democratic elements in place) since 1926 (way way more liberal than that of Soviet Union, or Nazi Germany). Before that it was fully democratic (though, chaotic).
As for the conflicts you'd decribed, some of them were conflicts without clear aggressor (e.g. Polish-Czechoslovak conflicts of 1918) typical of those times, some were misdeeds of Polish state (annexation of Vilnus and Czechoslovakian Zaolzie in 1938).
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).