The Red Army entered the eastern regions of Poland with seven field armies, containing between 450,000 and 1,000,000 troops, split between two fronts. Comandarm 2nd rank Mikhail Kovalyov lead the Red Army the invasion on the Belarusian Front while Comandarm 1st rank Semyon Timoshenko commanded the invasion on the Ukrainian Front.
Under the Polish defensive plan for the western border, Plan West, Poland assumed the Soviet Union would remain neutral. As a result, Polish commanders sent most of their troops to face the Germans invasion in the west. By this time only 20 under-strength battalions, consisted of about 20,000 troops of the Border Protection Corps, defended the eastern border. When the Red Army invaded, the Polish military was in the midst of a fighting retreat, with the intent of regrouping along the Romanian Bridgehead to await British and French relief.
Rydz-Śmigły, was initially inclined to order the eastern border forces to resist, but was dissuaded by Prime Minister Felicjan Sławoj Składkowski and President Ignacy Mościcki. At 4:00 pm on September 17 he issued an order commanding the troops to fall back and engage the Soviets only in self-defense. Communications systems had been severely damaged, breaking the chain of command. In the resulting confusion, clashes occurred along the border. General Wilhelm Orlik-Rückemann, given the command of KOP on August 30, had received no official directives since then, and he and his subordinates continued their armed resistance before dissolving the group on October 1.
The response of non-ethnic Poles to the situation added a further complication. Many Ukrainians, Belarusians and Jews welcomed the invading troops as liberators. The local reaction was mentioned by Lev Mekhlis, who told Stalin that people of West Ukraine welcomed the Soviets "like true liberators". The Organization of Ukrainian Nationalists rebelled against the Poles, and communist partisans organized local uprisings, such as that in Ski...