The invasion of Poland was a joint offensive on the Republic of Poland by Nazi Germany, the Slovak Republic, the Free City of Danzig, and the Soviet Union, which marked the beginning of World War II. The invasion began on 1 September 1939, when German, Slovak, and Danzig forces entered Poland. The Soviets invaded Poland on 17 September. The campaign ended on 6 October with Germany and the Soviet Union dividing and annexing the whole of Poland under the terms of the German–Soviet Frontier Treaty. The aim of the invasions was to disestablish Poland as a sovereign country, with its citizens destined for extermination.

The following is a timeline of the invasion, which includes events preluding to the offensives, battles and attacks during the invasion, before ending with the last Polish armed forces surrendering on 6 October, which then begins the Polish resistance movement against the German Military Administration in Poland and the Soviet Union occupational administration.

Prelude 15 June: The German Military High Command finalized the plans for an invasion of Poland, codenamed Operation Fall Weiss, also known as Operation White Castle.

17 August: Following German advances that started in early 1939, Vyacheslav Molotov, the Soviet Union Foreign Affairs Minister, after the failure to find a diplomatic agreement with the Allies, agreed to secretive diplomatic talks with the Germans. 21 August: Joachim von Ribbentrop, the German Minister of Foreign Affairs, arrives in Moscow for negotiations with Molotov. 23 August: The Treaty of Non-Aggression between Germany and the Union of Soviet Socialist Republics, commonly known as the Molotov–Ribbentrop Pact, was signed by Molotov and Ribbentrop. The treaty established a non-aggression pact between Nazi Germany and the Soviet Union, with a secret protocol establishing Soviet and German spheres of influence across Eastern Europe, including the division of Poland between the two countries. The treaty's existence was rumored to exist among the Allies throughout World War II, and was only proven to exist during the Nuremberg trials. 25 August: Adolf Hitler, dispatches orders to the Wehrmacht to invade Poland on 26 August following the plans of Case White. The United Kingdom signs an agreement with Poland, reaffirming mutual military assistance between the nations if either was attacked by some "European country", as agreed upon in the Anglo-Polish alliance. 26 August: Hitlers postpones the invasion of Poland after being "considerably shaken" (according to General Franz Halder) by the reaffirmation of the Anglo-Polish alliance. Approximately 70 German Abwehr agents, commanded by Lieutenant , attacked and failed to capture a key rail station and tunnel in the Jablunkov Pass in Poland. The attack, which became known as the Jabłonków incident, was part of Case White. Herzner was unaware Hitler had postponed the invasion at the last moment. The Jabłonków incident has been named the first commando operation of World War II. 28 August: German saboteur Anton Guzy planted and detonated two time bombs hidden in suitcases at the Tarnów train station in Poland, killing 20 people and injuring 35 others.

29 August: Polish Minister of Foreign Affairs Józef Beck ordered military mobilization, but cancelled the mobilization after pressure from the United Kingdom and France. 30 August: The Polish government orders general mobilization of the military again, to go into effect on 31 August. Poland launches Operation Peking, which successfully evacuates three destroyers of the Polish Navy, the , , and , to the United Kingdom. The Polish government announces that it has carried out defensive mining operations in its territorial waters. 31 August: Nazi sympathizer Edward VIII, the Duke of Windsor, sent a message to Victor Emmanuel III, the King of Italy, asking for Italy to intervene and try to prevent Germany and Poland from going to war. Hitler signs off and orders Heinrich Himmler, the leader of the Schutzstaffel (SS), to execute Operation Himmler, which consisted of several false flag operations to create various pretexts for an invasion of Poland. Under Operation Himmler, SS officers, led by Alfred Naujocks, wearing Polish military uniforms, attacked the Gleiwitz Radio Station in Gleiwitz in Germany (today Gliwice, Poland). Oskar Schindler, who is known for saving the lives of 1,200 Jews during the Holocaust, assisted the SS in the attack. Germany sends a last-minute ultimatum to Poland, which included 16 demands. Hitler later gave a speech announcing Poland rejected these demands before the invasion started, despite the ultimatum not being presented to Poland prior to the invasion.

September 1 September

Around 04:30 German Stukas III./KG1 dive bombers attack Polish sapper positions around the Tczew Road Bridge in the unsuccessful attempt to preempt the demolition of the bridge. This is widely recognized as the first attack of the invasion of Poland and subsequently World War II. At 04:45, the German battleship Schleswig-Holstein opens fire on the Polish garrison at Westerplatte, Free City of Danzig, starting the battle of Westerplatte. At 04:45, the Free City of Danzig Police began attacking the Polish Post Office in Danzig. At 04:45, the Luftwaffe targeted civilians during the bombing of Wieluń, which marked the first German war crime in World War II. At 05:00, approximately 50,000 Slovak soldiers cross the Polish border, beginning the Slovak invasion of Poland. At 05:00, the German Tenth Army, 31st Infantry Division, 1st and 4th Panzer Divisions crossed the Polish border and engaged the Polish Volhynian Cavalry Brigade and 7th Infantry Division near the village of Mokra. The subsequent battle of Mokra led to one of the few Polish victories during World War II, as confusion among the German lines led to the Germans firing on their own positions. Around 05:15, 225 German Marinestosstruppkompanie (marine shock-troops) and 1,500 Free City of Danzig Police engaged Polish defenders during the battle of Westerplatte. During the engagement, Polish Staff Sergeant Wojciech Najsarek was killed by machine-gun fire. Najsarek has been described as the first Polish combat casualty of the battle and perhaps of World War II. The German 3rd Army began attacking the town of Mława, which was defended by the Polish 20th Infantry Division. This attack began the German offensive known as the Battle of the Border. The German 4th Army advanced into the Polish Corridor, but was stopped by a Polish counterattack near Krojanty. The counterattack, known as the charge at Krojanty, was by the 18th Uhlans and gave birth to the myth of Polish cavalry attacking German tanks. The battle at Krojanty was one of the several engagements during the Battle of Tuchola Forest, where German forces were attempting to connect mainland Germany with East Prussia through the Tuchola Forest. The German 7th Infantry Division, under the command of Major General Eugen Ott, attacked fortified Polish defenses near Węgierska Górka. During the battle, the Germans outnumbers the Polish defenders 100 to 1, but suffered "heavy casualties" after breakthrough attempts were repelled. An eyewittness account of the battle was documented in From Lemberg to Bordeaux ('Von Lemberg bis Bordeaux'), written by Leo Leixner, a journalist and war correspondent. The German 5th Panzer Division began attacking the Polish 6th Infantry Division near Pszczyna amid the Battle of the Border. The Polish Navy launches Operation Worek, in which Poland's five submarines formed a screen in order to prevent German naval forces from carrying out landings on the Polish coast, and to attack enemy ships bombarding Polish coastal fortifications, in particular the base on the Hel Peninsula. The German military launches an assault on the Polish Hel Fortified Area, which would last until 2 October. At 11:00, several hours after the invasion of Poland began, Hitler gives a speech at the German Reichstag, announcing a declaration of war against Poland, using the false flag attacks during Operation Himmler and Poland declining Germany's ultimatum a few hours earlier as justifications for the invasion. The Einsatzgruppen, the death squads branch of the SS, begin Operation Tannenberg in Poland, which would kill around 20,000 selected Poles in two months. The Free City of Danzig is annexed by Germany. The House of Commons of the United Kingdom passes an emergency military budget, preparing for war against Germany. Polish Navy warships were attacked by German Luftwaffe aircraft in Gdańsk Bay (then Danzig Bay). The subsequent battle of Danzig Bay was the first naval-air battle of World War II.

2 September The Polish 16th Pomeranian Infantry Division counterattacks and recaptures lost territory during the battle of Grudziądz. The German 4th Army crosses the Brda River. In Stutthof, Germany, (formerly part of the Free City of Danzig), a prisoner-of-war camp for Polish prisoners is opened by the Germans. This camp eventually becomes Stutthof concentration camp in 1942. German troops conduct massacres in Bukownica, Gostyń, Łaziska Dolne, Łaziska Średnie, Torzeniec, Wyszanów, Parzymiechy and Zimnowoda, killing over 200 Poles, including women and children. The Luftwaffe bombed a train with civilian refugees from Krotoszyn in Koło, killing 300 people. Following the United Kingdom's lead, the French Parliament also approves an emergency war budget preparing for war against Germany. The British and French governments agree they will jointly issue an ultimatum to Germany.

3 September During the withdrawal of Polish troops from Bydgoszcz, local Germans opened fire on Polish soldiers and civilians, forcing them into a defensive battle in which several hundred people were killed on both sides. The event was referred to as the Bloody Sunday by the propaganda of Nazi Germany, with number of German victims upped to 58,000 by German propagandists. Bombing of Skierniewice by the Luftwaffe begins. German troops perpetrated massacres in Albertów, Imielin, Jankowice, Kobielice, Krzepice, Lędziny, Mysłów, Nierada, Pińczyce, Święta Anna, Zawiść, Zgoń and Zrębice, killing 320 Poles, including women and children. Soviet Defense Commissar Voroshilov orders seven military districts in the western Soviet Union to increase their combat readiness.

4 September Schleswig-Holstein intensifies its bombardment (supported through 21cm howitzers brought from East Prussia) against Polish defenders at Westerplatte as the Battle of Westerplatte continues. German 8th Army captures five bridges across the Warta river intact and begins its river crossing. Polish high command assigns the 41st Infantry Division and 44th Infantry Division, both still in deployment and unready for combat, to frontline duty to throw them into action. German troops carried out massacres in Cielętniki, Częstochowa, Kruszyna, Pasternik, Pławno, Pszczyna, Siewierz, and Sosnowiec, killing over 350 Poles, including women and children. Start of the evacuation of the Polish gold reserve from Warsaw to Lublin. The Luftwaffe bombed a train with civilian refugees from Ciechanów in Łukowo.

5 September

Edward Rydz-Śmigły orders a general withdrawal by Army Prusy, Army Kraków, Army Poznań and Army Łódź to the east bank of the Vistula and behind the Dunajec. Rydz-Śmigły orders a counterattack by Wyszków Operational Group with 1st Legions Infantry Division and 33rd Infantry Division at 18:15, though his order does not reach the Operational Group's commander Kowalski until 06:00 the next day. German troops perpetrated a massacre of over 80 Polish POWs in Serock (see German atrocities committed against Polish prisoners of war). Battle of Kazimierza Wielka between the Polish 55th Infantry Division and Wehrmacht, town captured by the Germans. During the night of 5/6 September, the Polish defenders of Różan evacuate their positions.

6 September