Soviet Union Violates Polish-Soviet Non-Aggression Pact with Invasion of Polish Territory by Red Army

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 Skidel. The Jewish population had suffered through pogroms in eastern Poland during the German invasion, and many saw the Soviets as the lesser of two evils. This reaction would strengthen the existing Polish fears of Żydokomuna and trouble Polish-Jewish relations into the 21st century.

The Polish political and military leaders knew that they were losing the war against Germany even before the Soviet invasion settled the issue. Nevertheless, they refused to surrender or negotiate a peace with Germany. Instead, the Polish government ordered all military units to evacuate Poland and reassemble in France. The government itself crossed into Romania at around midnight on 17 September 1939. Polish units proceeded to manoeuvre towards the Romanian bridgehead area, sustaining German attacks on one flank and occasionally clashing with Soviet troops on the other. In the days following the evacuation order, the Germans defeated the Polish Armies Kraków and Lublin at the Battle of Tomaszów Lubelski, which lasted from 17 September to 20 September.

Soviet units often met their German counterparts advancing from the opposite direction. Notable examples of co-operation occurred between the two armies in the field. The Wehrmacht passed the Brest Fortress, which had been seized after the Battle of Brześć Litewski, to the Soviet 29th Tank Brigade on 17 September. German General Heinz Guderian and Soviet Brigadier Semyon Krivoshein then held a joint victory parade in the town. Lwów (Lviv) surrendered on 22 September, days after the Germans handed the siege operations over to the Soviets. Soviet forces had taken Wilno on 19 September after a two-day battle, and they took Grodno on 24 September after a four-day battle. By 28 September, the Red Army had reached the line formed by the Narew, Western Bug, Vistula and San rivers—the border agreed in advance with the Germans.

Despite a tactical Polish victory on 28 September at the Battle of Szack, the outcome of the larger conflict was never in doubt. Civilian volunteers, militias, and reorganised retreating units held out against German forces in the in the Polish capital, Warsaw, until 28 September, and the Modlin Fortress, north of Warsaw, surrendered the next day after an intense sixteen-day battle. On 1 October, Soviet troops drove Polish units into the forests at the battle of Wytyczno, one of the last direct confrontations of the campaign.

Several isolated Polish garrisons managed to hold their positions long after being surrounded, such as those in the Volhynian Sarny Fortified Area which held out until September 25. The last operational unit of the Polish Army to surrender was General Franciszek Kleeberg's Independent Operational Group Polesie (Samodzielna Grupa Operacyjna "Polesie"). Kleeberg surrendered on 6 October after the four-day Battle of Kock (near Lublin), which ended the September Campaign. The Soviets were victorious. On 31 October, Molotov reported to the Supreme Soviet: "A short blow by the German army, and subsequently by the Red Army, was enough for nothing to be left of this ugly creature of the Treaty of Versailles".

The Soviet–Polish Non-Aggression Pact was an international treaty of non-aggression singed in 1932 by representatives of Poland and the USSR. The pact was unilaterally broken by the Soviet Union on September 17, 1939, during the Nazi and Soviet invasion of Poland.