Soviet aggression

The Polish defense was already broken, with their only hope being retreat and reorganisation in the south-eastern region (the Romanian Bridgehead), when on September 17, 1939, it was rendered obsolete overnight. The over 800,000 strong Soviet Union Red Army, divided into the Belarusian and Ukrainian fronts, invaded the eastern regions of Poland that had not yet been involved in military operations, 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 inhabiting Poland in view of Polish imminent collapse" while in reality the Soviets were acting in co-operation with Nazi Germany, carrying out their part of a secret deal (the division of Europe into Nazi and Soviet spheres of influence, as specified in the secret appendix of the Molotov-Ribbentrop Pact).

Polish border defence forces (Korpus Ochrony Pogranicza) in the east (about 25 battalions) were unable to defend the border, and Edward Rydz-Śmigły further ordered them to fall back and not engage the Soviets. This, however, did not prevent some clashes and small battles, like the defence of Grodno was defended by soldiers and local population. The Soviets murdered a number of Poles, including prisoners-of-war like General Józef Olszyna-Wilczyński. Ukrainians rose against the Poles, and communist partisans organised local revolts, e.g. in Skidel, 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 (near the Romanian border), while awaiting relief from a Western Allies attack on Germany's western border. Facing two powerful enemies, Nazi Germany and the Soviet Union, the Polish government decided that it was impossible to carry out the defence on Polish territories. However, it refused to surrender or negotiate for 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 17 September to 20 September, 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 22 September, and in a turn of events illustrating well this bizarre campaign, it capitulated to the Soviets, even though it had been attacked by Germans over a week earlier, in the middle of the siege, German troops handed 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 28 September. The Modlin Fortress north of Warsaw capitulated on 29 September 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 7 September, and Oksywie garrison held until 19 September. 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 2 October. 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 6 October, marking the end of the September Campaign.

Civilian losses

The Polish September Campaign was the start to one of the costliest total wars ever waged in Europe, with civilian casualties being high during and after the fighting. From the start of the campaign, the Luftwaffe attacked civilian targets and columns of refugees on the road to wreak havoc and disrupt the communications behind the Polish lines. The first such attack occurred at 4 AM on September 1 during the Bombing of Wieluń, in which ca. 1200 civilians were killed by a Luftwaffe air raid on Wieluń. Finally, apart from the victims of the battles, the German forces (both SS and the regular Wehrmacht) are credited with mass murder of several thousands of Polish POWs and civilians. Also, during a pre-planned Operation Tannenberg, circa 20,000 Poles were shot in 760 mass execution sites by special units, the Einsatzgruppen, in addition to regular Wehrmacht, SS and Selbstschutz.

Also, on September 3, 1939, "Bromberg Bloody Sunday," Polish Army units withdrawing through the city of Bromberg heard shots, supposedly from German fifth columnists firing at soldiers and civilians from rooftops and church towers. The Polish soldiers and civilians then lynched many of the alleged German saboteurs. Between 223 and 358 ethnic Germans were killed in Bromberg alone and more were killed in the surrounding villages. The exact number of the victims is still subject to dispute. In reprisal, German forces executed some 3,000 Poles and by the year's end sent an additional 13,000 to the Stutthof concentration camp.

Altogether, the civilian losses of Polish population amounted to 150,000 while German civilian losses amounted to circa 5,000.