Phase 2: Soviet aggression
Invasion of Poland: Germany and its allies from the west (blue), Soviets from the east (red).
From the beginning of the Polish campaign, the German government repeatedly asked Joseph Stalin and Molotov to act upon the August agreement and attack Poland from the east. Worried by an unexpectedly rapid German advance and eager to grab their allotted share of the country, Soviet forces attacked Poland on September 17. It was agreed that the USSR would relinquish its interest in the territories between the new border and Warsaw in exchange for inclusion of Lithuania in the Soviet "zone of interest." The USSR had openly supported German aggression, and Molotov stated after the Polish defeat: Germany, which has lately united 80 million Germans, has submitted certain neighboring countries to her supremacy and gained military strength in many aspects, and thus has become, as clearly can be seen, a dangerous rival to principal imperialistic powers in Europe - England and France. That is why they declared war on Germany on a pretext of fulfilling the obligations given to Poland. It is now clearer than ever, how remote the real aims of the cabinets in these countries were from the interests of defending the now disintegrated Poland or Czechoslovakia.
By September 17, 1939, the Polish defense was already broken, and their only hope was to retreat and reorganize along the Romanian bridgehead. However, these plans were rendered obsolete nearly overnight, when the over 800,000 strong Soviet Union Red Army entered and created the Belarussian and Ukrainian fronts after invading the eastern regions of Poland. This was in violation of the Riga Peace Treaty, the Soviet-Polish Non-Aggression Pact, and other international treaties, both bilateral and multilateral. Soviet diplomacy claimed that they were "protecting the Ukrainian and Belarusian minorities of eastern Poland in view of Polish imminent collapse. Vyacheslav Molotov delivered a speech on September 17, 1939:
Events arising out of the Polish‑German War has revealed the internal insolvency and obvious impotence of the Polish state. Polish ruling circles have suffered bankruptcy. . . . Warsaw as the capital of the Polish state no longer exists. No one knows the whereabouts of the Polish Government. The population of Poland have been abandoned by their ill‑starred leaders to their fate. The Polish state and its government have virtually ceased to exist. In view of this‑state of affairs, treaties concluded between the Soviet Union and Poland have ceased to operate. A situation has arisen in Poland which demands of the Soviet‑Government especial concern for the security of its state. Poland has become a fertile field for any accidental and unexpected contingency that may create a menace to the Soviet Union. . . . Nor can it be demanded of the Soviet Government that it remain indifferent to the fate of its blood brothers, the Ukrainians and Byelorussians [White Russians] inhabiting Poland, who even formerly were without rights and who now have been abandoned entirely to their fate. The Soviet Government deems it its sacred duty to extend the hand of assistance to its brother Ukrainians and brother Byelorussians inhabiting Poland.
Polish border defence forces in the east, known as the Korpus Ochrony Pogranicza, consisted of about 25 battalions. Edward Rydz-Śmigły ordered them to fall back and not engage the Soviets. This, however, did not prevent some clashes and small battles, like the Battle of Grodno, as soldiers and local population attempted to defend the city. The Soviets murdered numerous Poles, including prisoners of war like General Józef Olszyna-Wilczyński. Organization of Ukrainian Nationalists rose against the Poles, and communist partisans organised local revolts, robbing and murdering Poles. Those movements were quickly disciplined by the NKVD. The Soviet invasion was one of the decisive factors that convinced the Polish government that the war in Poland was lost. Prior to the Soviet attack from the east, the Polish military's fall-back plan had called for long-term defence against Germany in the southern-eastern part of Poland, while awaiting relief from a Western Allies attack on Germany's western border. However, the Polish government refused to surrender or negotiate a peace with Germany and ordered all units to evacuate Poland and reorganize in France.
Meanwhile, Polish forces tried to move towards the Romanian bridgehead area, still actively resisting the German invasion. From September 17 to September 20, the Polish Armies Kraków and Lublin were crippled at the Battle of Tomaszów Lubelski, the second largest battle of the campaign. The city of Lwów capitulated on September 22 because of Soviet intervention; the city had been attacked by the Germans over a week earlier, and in the middle of the siege, the German troops handed operations over to their Soviet allies. Despite a series of intensifying German attacks, Warsaw—defended by quickly reorganised retreating units, civilian volunteers and militia—held out until its capitulation on September 28. The Modlin Fortress north of Warsaw capitulated on September 29 after an intense 16-day battle. Some isolated Polish garrisons managed to hold their positions long after being surrounded by German forces. Westerplatte enclave's tiny garrison capitulated on September 7, and Oksywie garrison held until September 19; Hel was defended until October 2.
Despite a Polish victory at the battle of Szack, after which the Soviets executed all the NCOs and officers they had managed to capture, the Red Army reached the line of rivers Narew, Western Bug, Vistula and San by September 28, in many cases meeting German units advancing from the other side. Polish defenders on the Hel peninsula on the shore of the Baltic Sea held out until October 2. The last operational unit of the Polish Army, General Franciszek Kleeberg's Samodzielna Grupa Operacyjna "Polesie", capitulated after the 4-day Battle of Kock near Lublin on October 6, marking the end of the September Campaign.
