The Polish resistance movement in World War II was arguably the largest underground resistance in Nazi-occupied Europe. *
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The main organization was the Armia Krajowa (AK) - Home Army, loyal to the government in exile in London. 
It counted up to 400,000, although a number of 600.000 is also quoted.
It was the military arm of the Polish Underground State. It began as the ZWZ (Zwizek Walki Zbrojnej - Union of Armed Struggle) created in November 1939. After incorporating other minor organisations it was reformed to the Home Army in Fenruary 1942.
Most other Polish underground armed organizations were incorporated or subordinated to the Home Army (though they retained varying degrees of autonomy), 
the largest of them were Bataliony Chlopskie (Peasants' Battalions) and part of NSZ (National Armed Forces).*
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The Home Army sabotaged German operations such as transports headed for the Eastern Front. 
It also fought several full-scale battles against the Germans, particularly in 1943 and in Operation Tempest in 1944. 
The biggest Home Army operation was the 1944 Warsaw Uprising. *
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The second large organization was Bataliony Chlopskie (BCh) - Peasants' Battalions, formed by leftist agrarian political party, also loyal to the government in exile. It counted up to 160,000.*
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Behind the mainstream were nationalist NSZ, Jewish ZOB, and communist GL/AL.*
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