[Lisbon1940]

USSR entered Poland September 17, 1939, more than two weeks after Germany attacked on September 1.

[Trapped Behind Enemy Lines]

Yes and that was a brilliant move on the part of Stalin. The Germans were thus tasked with all of the heavy lifting of destroying the Polish armed forces. By the time Stalin moved his divisions into eastern Poland, the country was already in shambles.

[FredZarguna]

USSR entered invaded Poland September 17, 1939, more than two weeks after Germany attacked on September 1 as soon as it was clear to Stalin he wouldn't need to fight anybody.