The Croatian Partisans, officially the National Liberation Movement in Croatia (; NOP), were part of the Communist-led Yugoslav Partisans, a guerrila force in the Axis-occupied Yugoslavia that fought against the Axis-powers forces and Serbian royalists from July 1941 until May 1945 during World War II, across the territory of the Axis-allied Independent State of Croatia (NDH). Croatian Partisan units succeeded in temporarily or permanently liberating considerable parts of Croatia from Axis forces. The defeat of the NDH in mid-May 1945 was followed by mass killings of Croat prisoners and others at the hands of the Communist forces and the establishment later that year of a pro-Soviet (until mid-1948) one-party Communist Federal People's Republic of Yugoslavia, the People's Republic of Croatia being one of its constituent republics.