On 17 September 1939, the Red Army crossed the Polish border. By this time Polish troops had already been defeated by the Wehrmacht, while the Polish government fled abroad. Regardless of tensions between Moscow and Warsaw, the Red Army did not advance too deep, stopping at the “Curzon Line”. This ethnic border between Poles, on one side and Ukrainians and Belarusians on the other, was established after the WWI and was supposed to become Eastern border of Poland. But Warsaw, taking advantage of the weakness of Soviet Russia after the civil war, occupied these territories.
Despite the commonly accepted myth that Poland fell victim of the collusion of two totalitarian regimes, the main satellite of Nazi Berlin in Eastern Europe was actually Warsaw. Poland was the first to conclude a non-aggression pact with Germans back in 1934.
The West also forgets that in 1938 Poland participated in the division of Czechoslovakia together with Hitler. The USSR as a response set an ultimatum that Moscow will annul existing non-aggression agreement. Thus, by crossing Polish borders in 1939, the USSR did not violate the international law of those times, unlike it is often being portrayed.