Invasion of Poland

At 04:40 on 1 September 1939 German aircraft struck the town of Wieluń and the battleship Schleswig-Holstein opened fire on Polish positions at Danzig, opening the first campaign of the Second World War. Army Group North under Fedor von Bock attacked out of Pomerania and East Prussia to sever the Polish Corridor and drive on Warsaw, while Army Group South under Gerd von Rundstedt struck from Silesia and Slovakia with the 8th, 10th and 14th Armies. Sixty-two German divisions, backed by more than 1,300 aircraft, faced a Polish army of roughly thirty active divisions deployed, on Marshal Edward Rydz-Śmigły's insistence, too close to the frontier to trade space for time.

The Polish air force had moved to hidden airfields before the war and kept flying into the second week, but it could not contest the Luftwaffe for long. German panzer spearheads, notably Erich Hoepner's XVI Corps, broke through where the Łódź and Kraków Armies met, while Heinz Guderian's XIX Corps cut across the Polish Corridor. Britain and France declared war on Germany on 3 September but took no direct military action to relieve the pressure. By the second week the German 10th Army under Walter von Reichenau was closing on Warsaw from the southwest, and Army Group North's 3rd and 4th Armies had linked up across the Corridor and were pressing south toward the capital from East Prussia.

Polish forces attempted the campaign's largest counterstroke between 9 and 19 September, when the retreating Poznań and Pomorze Armies under Tadeusz Kutrzeba struck the flank of the German 8th Army along the Bzura river west of Warsaw. The attack achieved early tactical surprise and inflicted substantial losses, but German reinforcement and air attack encircled the Polish force; the survivors who escaped fell back into the Warsaw and Modlin garrisons. Warsaw itself came under siege from mid-September, enduring heavy bombing and artillery bombardment while its garrison and civilian volunteers held out under General Walerian Czuma.

On 17 September the Soviet Union invaded from the east under the secret protocols of the Molotov–Ribbentrop Pact, sending the Belorussian Front under Mikhail Kovalyov and the Ukrainian Front under Semyon Timoshenko across a border already stripped of most Polish defenders. Soviet forces met scattered resistance, most notably at Grodno, while the Polish government and high command crossed into Romania. Caught between two invaders, organized Polish resistance in the interior collapsed. Warsaw, under Juliusz Rómmel and the city's civil defence, surrendered on 27–28 September after prolonged bombardment; the Modlin fortress fell on 29 September, and the naval base at Hel held out until 2 October.

The campaign's last organized force in the field was the Independent Operational Group Polesie under General Franciszek Kleeberg, built around the 50th and 60th Infantry Divisions, the Podlaska Cavalry Brigade and the improvised Zaza Cavalry Division. Marching to relieve Warsaw and finding it too late, Kleeberg turned his column toward Kock, southeast of the capital, where it ran into the German XIV Motorised Corps of the 10th Army in early October. In the Battle of Kock, fought 2–5 October, Polish troops threw back the first German attacks on Kock and Serokomla and, on 3 October, counterattacked the German 13th Motorised Division, gaining ground before the attack stalled west of Wola Gułowska. The German 29th Motorised Division joined the fighting on 4–5 October, and Kleeberg, surrounded and out of ammunition and food, surrendered his remaining troops on 6 October 1939, ending organized Polish resistance.

Poland was partitioned between Germany and the Soviet Union, with a small area later ceded to Lithuania and Slovakia retaining border gains in the south. Losses on both sides were heavy relative to the campaign's length, and the invasion set the pattern of occupation, deportation and mass killing that followed across German- and Soviet-held Poland.