The Polish Home Army (Armia Krajowa) (circa 1942--1944) | Los Angeles Museum of the Holocaust

The Armia Krajowa (Polish pronunciation: [ˈarmja kraˈjɔva], abbreviated AK), or Home Army, was the dominant Polish resistance movement in World War II German-occupied Poland. It was formed in February 1942 from the Związek Walki Zbrojnej (Union for Armed Struggle). Over the next two years, it absorbed most other Polish underground forces. It was loyal to the Polish government in exile and constituted the armed wing of what became known as the "Polish Underground State." Estimates of its membership in 1944 range from 200,000 to 600,000, with the most common number being 400,000; that figure would make it not only the largest Polish underground resistance movement but one of the three largest in Europe during World War II. It was disbanded on January 20, 1945, when Polish territory had been mostly cleared of German forces by the advancing Soviet Red Army.

The AK's primary resistance operations were the sabotage of German activities, including transports headed for the Eastern Front in the Soviet Union. The AK also fought several full-scale battles against the Germans, particularly in 1943 and 1944 during Operation Tempest. They tied down significant German forces, diverting much-needed supplies, while trying to support the Soviet military.

The most widely known AK operation was the failed Warsaw Uprising. The AK also defended Polish civilians against atrocities committed by German and non-German (collaborationist) military organizations. Due to its ties with the Polish government in exile, the Armia Krajowa was viewed by the Soviet Union as a major obstacle to its takeover of the country. There was increasing conflict between AK and Soviet forces both during and after the war.