The Croatian Partisans, officially the National Liberation Movement in Croatia (Croatian: Narodnooslobodilački pokret u Hrvatskoj; NOP), were part of the anti-fascist National Liberational Movement in the Axis-occupied Yugoslavia which was the most effective anti-Nazi resistance movement. It was led by Yugoslav revolutionary communists during the World War II. NOP was under the leadership of the League of Communists of Yugoslavia (KPJ) and supported by many others, with Croatian Peasant Party members contributing to it significantly. NOP units were able to temporarily or permanently liberate large parts of Croatia from occupying forces. Based on the NOP, the Federal Republic of Croatia was founded as a constituent of the Democratic Federal Yugoslavia.

In April 1941, the Croatian people found itself once again in a position of solving the issue of Croatian survival in the swirling of international warfare. Vladko Maček, leader of HSS and at the time de facto political leader of Croats, estimated that the Croatian state had no real possibility for surviving as part of Nazi Germany's war reconstruction of Europe so he refused to declare an independent Croatian state within the Axis system. Convinced that the Axis powers would lose the war and that their totalitarian system was not aligned with HSS's ideas of liberal democracy and peacemaking, Maček tried in all ways, including entering the Yugoslav government-in-exile, to preserve the changes that had been made within the Kingdom of Yugoslavia and to protect the Croatian people from bloodshed. However, when the war reached Croatian territory, prevented by Ustashe police control, Maček opted for a policy of waiting to see how the things would turn, left the political scene and handed it over to the Ustaše and the Communists.

Nazi-puppet state, Independent State of Croatia (NDH) proclaimed by Slavko Kvaternik in the name of Ustaša leader Ante Pavelić on 10 April 1941 appeared as a discontinuity in relation towards the approximation of the basic line of Croatian political orientation and a failure of the aspiration of the Croatian people to have an independent state because NDH's existence was directly linked to the will and destiny of Nazi Germany. The borders of NDH included Bosnia and Herzegovina and parts of Syrmia, but not Međimurje, Istria and large parts of Dalmatia (which were given to Italy with 1941 Treaties of Rome). With Treaties of Rome, NDH was proclaimed the kingdom, and the crown was offered to a member of the Italian ruling dynasty, Prince Aimone, Duke of Aosta as Tomislav II. NDH's constitution was based on a totalitarian ideology that developed under the influence of Nazism and Fascism.

Racial laws were soon enacted and the Ustaše targeted Serbs, Roma and Jews for extermination. Anti-fascist Croats were also persecuted by the regime. Dissatisfaction of the Croatian people with Ustaše rule started almost immediately with the beginning of these persecutions.

The NDH was not truly independent in relation towards German and Italian occupation authorities and with large parts of its territory being controlled by Chetniks (parts of Dalmatian Hinterland, Lika, Bosnia and others) and growing Partisan movement. The significance of the regime and the German and Italian influence did not leave much room for independent activity in any area of social life. With the Lorković–Vokić plot in summer of 1944, high-ranking Ustasha officials unsuccessfully tried to preserve NDH by taking power and switching sides to the Allies.

Communist activity was aimed at preserving Yugoslavia and its transformation into a federal multiethnic communist state. That is why the KPJ's (in which KPH was active as a special organization since 1937) basic political position was the gathering of all political groups and people ready to provide resistance to the occupying forces and collaborators. In Croatian territories, that meant primarily to win over the Croatian population that had until then followed the HSS and to stop the strengthening of the Chetnik movement among the Serbian population and eventually unite them into a broad anti-fascist movement.

First Armed Anti-fascist Resistance Unit in Europe was founded by a group of Croats and one notable Serb woman, Nada Dimić, in the forest of Žabno near Sisak on 22 June 1941 under the leadership of Vlado Janić-Capo. Partisans in Croatia wore three-cornered caps like International Brigades in the Spanish Civil War, called Triglavka. Their goal was, first and foremost, to liberate Croatia from the German and Italian occupation and terror which was conducted by the Ustaše regime against Jews, Romanis, Serbs, Croats and others who did not accept their principles. Soon afterward, Croatian partisans founded a Main Staff of the Croatian Partisans (Croatian: Glavni štab NOV i PO Hrvatske) led by Andrija Hebrang which was a part of the Supreme Staff of the Yugoslav Partisans under the command of Josip Broz Tito. Of all the other main staffs in the territory of Yugoslavia, Croatian was the strongest and most developed operational-territorial body of the Partisan forces, both in terms of the number of staff and the duties that it had. Following the unsuccessful uprising in Serbia in 1941, the center of gravity for the resistance moved to Bosnia and Herzegovina and Croatia.

On April 13, 1941, Winston Churchill sent his greetings to the Yugoslav people. In his greeting he stated: