In World War II, the Polish armed forces were the fourth largest Allied forces in Europe, after those of the Soviet Union, United States and Britain. Poles made substantial contributions to the Allied effort throughout the war, fighting on land, sea, and in the air.

Polish forces in the east, fighting alongside the Red army and under Soviet high command, took part in the Soviet offensives across Belarus and Ukraine into Poland and across the Vistula and Oder Rivers to the Battle of Berlin.

In the west, Polish paratroopers from the 1st Independent Polish Parachute Brigade fought in the Battle of Arnhem / Operation Market Garden; while ground troops were present in the North Africa Campaign (siege of Tobruk); the Italian campaign (including the capture of the monastery hill at the Battle of Monte Cassino); and in battles following the invasion of France (the battle of the Falaise pocket; and an armored division in the Western Allied invasion of Germany).

Particularly well-documented was the service of 145 Polish pilots flying British planes under British Command during the Battle of Britain, 79 in mixed squadrons under the RAF after July 1940, 32 in wholly Polish Squadron 303 after 31 August 1940 and 34 in entirely Polish Squadron 302. Other instances of service flying French planes in the Polish Air Force took place during the Battle of Britain at the same time, and from 1944 the Polish Air Force (also with British planes) was established in Britain.

Some Polish contributions were less visible, notably the prewar and wartime decrypting of German Enigma-machine ciphers by cryptologists Marian Rejewski, Henryk Zygalski, and Jerzy Różycki. An extensive Polish intelligence network also proved of great value to Allied intelligence.

The European Theatre of World War II opened with the German invasion of Poland on Friday September 1, 1939, followed by the Soviet invasion of Poland on September 17, 1939. On 6 October, following the Polish defeat at the Battle of Kock, German and Soviet forces gained full control over Poland. The success of the invasion marked the end of the Second Polish Republic, though Poland never formally surrendered. A Polish Underground State with a government-in-exile that would eventually set up headquarters in London resumed the struggle against the occupying powers. The Polish forces in the West, as well as in the East and an intelligence service were established outside of Poland, and contributed to the Allied effort throughout the war.

Invasion of Poland

The invasion of Polish Second Republic by the military forces of Nazi Germany on September 1, 1939, marked the beginning of World War II in Europe. The Soviets invaded Poland on September 17 as had been agreed with Nazi Germany in the Molotov-Ribbentrop Pact.

In keeping with the terms of the Secret Additional Protocol of the Molotov–Ribbentrop Pact Germany informed the Soviet Union that its forces were nearing the Soviet interest zone in Poland and so urged the Soviet Union to move into its zone. The Soviets had been taken by surprise by the speed of the German advance as they had expected to have several weeks to prepare for an invasion rather than merely a few days. They did promise to move as quickly as possible. On September 17 the Soviets invaded eastern Poland, forcing the Polish government and military to abandon their plans for a long-term defense in the Romanian bridgehead area. The last remaining Polish Army units capitulated in early October.

In accordance with their treaty obligations, the United Kingdom and France declared war on Germany on September 3. Hitler had gambled, incorrectly, that France and Britain would allow him to annex parts of Poland without military reaction. The campaign began on September 1, 1939, one week after the signing of the Molotov–Ribbentrop Pact containing a secret protocol for the division of Northern and Central Europe into German and Soviet spheres of influence. It ended on October 6, 1939, with Germany and the Soviet Union occupying the entirety of Poland.

German losses included about 16,000 killed in action, 28,000 wounded, 3,500 missing, over 200 aircraft, and 30% of their armored vehicles. The Polish casualties were about 66,000 dead and 694,000 captured.

German losses in the Polish campaign amounted to 50% of all casualties they would suffer until their invasion of USSR in 1941; and the campaign that lasted about a month consumed eight months worth of supplies.

Aid to Jews

A substantial number of Poles risked their lives in the German occupation to save Jews. German-occupied Poland was the only European territory where the Germans punished any kind of help to Jews with death for the helper and his entire family. Even so, Poland was also the only German-occupied country to establish an organization specifically to aid Jews. Known by the cryptonym Żegota, it provided food, shelter, medical care, money, and false documents to Jews. Most of Żegota's funds came directly from the Polish Government-in-Exile in Great Britain.

Most Jews who survived the German occupation of Poland were saved by Poles unconnected with Żegota. Estimates of Jewish survivors in Poland range from 40,000 to 50,000 to 100,000–120,000. Scholars estimate that it took the work of ten people to save the life of one Polish Jew. Of the individuals awarded medals of Righteous among the Nations (given by the State of Israel to non-Jews who saved Jews from extermination in the Holocaust) those who were Polish citizens number the greatest. There are 6,339 Polish men and women recognized as "Righteous" to this day, amounting to over 25 percent of the total number of 22,765 honorary titles awarded already.

Polish resistance

The main resistance force in German-occupied Poland was the Armia Krajowa ("Home Army"; abbreviated "AK"), which numbered some 400,000 fighters at its peak as well as many more sympathizers. Throughout most of the war, AK was one of the three largest resistance movements in the war. The AK coordinated its operations with the exiled Polish Government in London and its activity concentrated on sabotage, diversion and intelligence gathering. Its combat activity was low until 1943 as the army was avoiding suicidal warfare and preserved its very limited resources for later conflicts that sharply increased when the Nazi war machine started to crumble in the wake of the successes of the Red Army in the Eastern Front. Then the AK started a nationwide uprising (Operation Tempest) against Nazi forces. Before that, AK units carried out thousands of raids, intelligence operations, bombed hundreds of railway shipments, participated in many clashes and battles with the German police and Wehrmacht units and conducted tens of thousands of acts of sabotage against German industry The AK also conducted "punitive" operations to assassinate Gestapo officials responsible for Nazi terror. Following the 1941 German attack on the USSR, the AK assisted the Soviet Union's war effort by sabotaging the German advance into Soviet territory and provided intelligence on the deployment and movement of German forces. After 1943, its direct combat activity increased sharply. German losses to the Polish partisans averaged 850–1,700 per month in early 1944 compared to about 250–320 per month in 1942.

In addition to the Home Army, there was an underground ultra-nationalist resistance force called Narodowe Siły Zbrojne (NSZ or "National Armed Forces"), with a fiercely anti-communist stance. It participated in fighting German units, winning many skirmishes. From 1943 onwards, some units took part in battling the Gwardia Ludowa and the Polish People's Army PAL, both communist resistance movement. From 1944, the advancing Red Army was also seen as a foreign occupation force, prompting skirmishes with the Soviets as well as Soviet-backed partisans. In the later part of the war, when Soviet partisans started attacking Polish partisans, sympathizers and civilians, all non-communist Polish formations were (to an increasing extent) becoming involved in actions against the Soviets.

The Armia Ludowa, a Soviet proxy fighting force was another resistance group that was unrelated to the Polish Government in Exile, allied instead to the Soviet Union. As of July, 1944 it incorporated a similar organization, the Gwardia Ludowa and the Polish People's Army PAL, and numbered about 6,000 soldiers (although estimates vary).

There were separate resistance groups organized by Polish Jews: the right-wing Żydowski Związek Walki ("Jewish Fighting Union") (ŻZW) and the more Soviet-leaning Żydowska Organizacja Bojowa ("Jewish Combat Organization") (ŻOB). These organisations cooperated little with each other and their relationship with the Polish resistance varied between occasional cooperation (mainly between ZZW and AK) to armed confrontations (mostly between ŻOB and NZS).

Other notable Polish resistance organizations included the Bataliony Chłopskie (BCh), a mostly peasant-based organization allied to the AK. At its height the BCh included 115,543 members (1944; with additional LSB and PKB-AK Guard, for the estimated total of 150,250 men, not confirmed).

Throughout the war the German state was forced to divert a substantial part of its military forces to keep control over Poland:

+ (does not include annexed territories of Poland and parts of Kresy) Period Wehrmacht Police and SS Total October 1939 550,000 80,000 630,000 April 1940 400,000 70,000 470,000 June 1941 2,000,000 50.000 2,050,000 February 1942 300,000 50,000 350,000 April 1943 450,000 60,000 510,000 November 1943 550,000 70,000 620,000 April 1944 500,000 70,000 570,000 September 1944 1,000,000 80,000 1,080,000

+ and Home Army (AK) from 1 January 1941 to 30 June 1944Action typeAction totals Damaged locomotives6,930Delayed repairs to locomotives803Derailed transports732Transports set on fire443Damage to railway wagons19,058Blown up railway bridges38Disruptions to electricity supplies in the Warsaw grid 638Army vehicles damaged or destroyed4,326Damaged aeroplanes28Fuel tanks destroyed1,167Fuel destroyed (in tonnes)4,674Blocked oil wells5Wagons of wood wool destroyed150Military stores burned down130Disruptions of production in factories7Built-in faults in parts for aircraft engines4,710Built-in faults into cannon muzzles203Built-in faults into artillery projectiles92,000Built-in faults into air traffic radio stations107Built-in faults into condensers70,000Built-in faults into (electro-industrial) lathes1,700Damage to important factory machinery2,872Various acts of sabotage performed25,145Planned assassinations of Germans5,733

Intelligence

Polish intelligence supplied valuable intelligence to the Allies; 48% of all reports received by the British secret services from continental Europe in between 1939 and 1945 came from Polish sources. The total number of those reports is estimated at 80,000, and 85% of them were deemed high or better quality. Despite Poland becoming occupied, the Polish intelligence network not only survived but grew rapidly, and near the end of the war had over 1,600 registered agents (Another estimate gave about 3,500).

Western Allies had limited intelligence assets in Central and Eastern Europe, and extensive Polish intelligence network in place proved to be a major asset, even described as "the only allied intelligence assets on the Continent" following the French capitulation. According to , for the Western Allies, the intelligence provided by the Home Army was considered to be the best source of information on the Eastern Front.