[qbaqbaqba]

USSR didn't exist in 1920 either, it was established in 1922.

[vetinari]

True, but when you use the optics "it is no man's land, let's take it while we can", why only in the case of 1919-1921, but not in the 1939 case?

When Soviets crossed Polish borders in 1939, the Polish statehood was exactly in the same defunct state as the Soviet in 1919-21.

[jagger11]

Polish borders as of 1918 are here - https://pl.wikipedia.org/wiki/Historia_Polski_(1918%E2%80%93...

Soviet Union decided to attack, it lost the war, it had to ceede territory.

[qbaqbaqba]

Poles didn't cross the Soviet border in 1919.