From the beginning, the German government repeatedly asked Joseph Stalin to act upon the Molotov-Ribbentrop Pact and attack Poland from the east and on 17 September, the Red Army invaded Poland. The Soviet government announced that it was acting to protect the Ukrainians and Belarusians who lived in the eastern part of Poland, because the Polish state had collapsed in the face of the German attack and could no longer guarantee the security of its own citizens.
The Red Army entered the eastern regions of Poland with seven field armies, almost 1,000,000 troops.
The Polish Army originally had a well-developed defensive plan to deal with the threat of the Soviet Union, but they were unprepared to face two invasions at once. By the time the Soviets invaded, the Polish commanders had sent most of their troops west to face the Germans, leaving the east protected by only 20 under-strength battalions.
At first, the Polish commander-in-chief, Marshal of Poland Edward Rydz-Śmigły, ordered the border forces to resist the Soviets. He then changed his mind after consulting with Prime Minister and ordered them to fall back and engage the Soviets only in self-defense.
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