It wasn't an aggressive war at least for two reasons:
1) Poland attacked Russia's ally in 1938 - Czechoslovakia and by that cancelled Soviet-Polish prace treaty, so it was collective self-defense.
2) Polish state didn't exist anymore when Soviet state came on the previously occupied by Poland lands of Western Ukraine and Western Belarus. One can't kill what is already dead.
Weak justifications for a joint nazi-soviet invasion of Poland.