It was only a few days later after Great Britain and France had declared war on Germany, that the Soviet Union took part on the debacle, and in September 17 1939, they invaded Poland and the Soviet Foreign Minister Vyacheslav Molotov declared that Poland as an independent form of government stopped existing - this was part of the Molovotv-Ribbentrop agreement where they signed a non-aggression pact between both countries to then each control a part of Poland.
The country was already quite a mess because Germany's troops had invaded Poland 17 days before and most people were fleeing east (Germany was attacking and occupying from the west) only to meet their doom on the east a few days later in the hands of the Soviets.
It was known until 1990 that the URSS signed the non-aggression pact because they used the argument that that Russia had to come to the aid of its “blood brothers,” the Ukrainians and Byelorussians, who were trapped in territory that had been illegally annexed by Poland.
As Soviet troops broke into Poland, they unexpectedly met up with German troops who had fought their way that far east in a little more than two weeks. The Germans receded when confronted by the Soviets, handing over their Polish prisoners of war. Thousands of Polish troops were taken into captivity; some Poles simply surrendered to the Soviets to avoid being captured by the Germans.
