Poland was the only country to combat Germany from the first day of the Polish invasion until the end of the war in Europe. After the disaster of September 1939, a constitutionally legitimate Polish government-in-exile established a seat in London under the direction of General Wladyslaw Sikorski. In the early years of the war, Stalin maintained a strained cooperation with the Polish government-in-exile while continuing to demand retention of the eastern Polish territories secured by the Hitler-Stalin pact and assurances that postwar Poland would be "friendly" toward the Soviet Union.

Shortly after Germany invaded the Soviet Union, the Kremlin sought to organize Polish forces to aid in repelling the Nazis on the Eastern Front. Although 75,000 Polish troops were amassed on Soviet soil from Soviet camps, they never were deployed on the Soviet front because of disagreements about their utilization. Instead, the forces under the command of the "London Poles" fought with great distinction in the British Eighth Army in North Africa and Italy. The armored Polish I Corps played an important role in the Normandy invasion. Although some Polish units fought with the Red Army on the Eastern Front in the early years of the war, by 1943 Stalin had broken relations with the Sikorski government and the Soviet Union formed a rival front group, the Union of Polish Patriots, led by Polish communists in the Soviet Union. That group formed an entire field army that aided the Red Army in the last year of the war.

Polish intelligence personnel also made a major contribution to the Allied side. In the 1930s, Polish agents had secured information on the top-secret German code machine, Enigma, and in the war émigré Polish experts aided the British in using this information to intercept Hitler's orders to German military leaders.

In Poland itself, most elements of resistance to the German regime organized under the banner of the Home Army (Armia Krajowa), which operated under direction of the London government-in-exile. The Home Army became one of the largest and most effective underground movements of World War II. Commanding broad popular support, it functioned both as a guerrilla force, conducting a vigorous campaign of sabotage and intelligence gathering, and as a means of social defense against the invaders. The Home Army became the backbone of a veritable underground state, a clandestine network of genuine Polish institutions and cultural activities. By 1944 the Home Army claimed 400,000 members. Acting independently of the overall Polish resistance, an underground Jewish network organized the courageous but unsuccessful 1943 risings in the ghettos of Warsaw, Bialystok, and Vilnius.

Data as of October 1992