On 17 September 1939, Soviet troops crossed the frontier into Poland and within a few days were in control of around half the geographical area of the Polish state. The speed of the Soviet advance was mainly attributable to the absence of Polish resistance. For seventeen days the Polish Army had been putting up a stubborn, though unsuccessful, defence against the German invasion. Pushed back towards Galicia, the Poles were concentrating all their military efforts against the German Army, leaving only handfuls of frontier guards on their eastern border with the Soviet Union. Facing west and already under severe pressure, the Polish Army was in no position to defend Poland against the invading forces of two major powers. Moreover, the Polish Government was under the illusion that the Polish–Soviet non-aggression pact of 1932 would protect Poland against a Soviet attack. Hence there was a brief misapprehension among the population of the Polish eastern provinces that the Red Army was marching to their assistance against the Germans. The Soviet advance was therefore welcomed by all sections of the population. The Ukrainian, Belorussian, and Jewish elements, however, hoped that the Soviet troops would liberate them from the Polish state, which they thought of as oppressive.

This fourth partition of Poland was a direct consequence of the German–Soviet non-aggression pact of 23 August 1939, which contained a secret protocol providing for the delimitation of German and Soviet spheres of interest in eastern Europe.