At the onset of World War II, on 1 September 1939 Nazi Germany invaded Poland. Polish forces were overwhelmed by the German attack in September 1939, which was followed on 17 September 1939 by an invasion by the Soviet Union. Some Polish forces escaped from the occupied country and joined Allied forces fighting in other theaters while those that remained in Poland splintered into guerilla units of the "Armia Krajowa" ("Home Army") and other partisan groups which fought in clandestine ways against the foreign occupiers. Thus, there were three threads to Polish armed forces from 1939; the Polish Armed Forces in the West, the "Armia Krajowa" and other resistance organizations fighting the Germans in Poland, and the Polish Armed Forces in the East, which later became the post-war communist Polish People's Army (LWP).