Poland invaded the former Russian Empire during the russian civil war. It annexed large areas.

The USSR tried to revive the Entente for years, but was rebuffed by France, GB and the US. Partially because local fascists, partially because these nations had no real interest in a new entente, partially because Poland lobbied against the USSR.

Then the Munich conference happened. Previously the USSR was the only nation willing to military support Czechslovakia, but was denied military access by other countries to get there. Czechslovakia ceased to exist and was carved up by its neighbours.

This made it clear to the USSR that their dream of a new Entente had ended and the western capitalist nations wanted to direct germany eastwards against them. So they did two things:

1. Sign a non-agression-treaty with germany. As the last european state to do so. This lead Germany to move west first.
2. Offer Poland a defensive treaty. Poland denied, because of anti-soviet tendency and pro-german one.

So when Germany invaded Poland, the USSR did fuck all for two weeks. Then, after the polish government had left for exile, they moved to the territories Poland had annexed previously and occupied them. The Polish governments last order before exile were not to resist the Red Army.