Seeking retribution for their withdrawal from the Tripartite Pact after the March 1941 Yugoslav coup d'état, Adolf Hitler sought to destroy the Kingdom of Yugoslavia by dismembering it for annexation by Nazi Germany and its allies.

[4] Pavelić and the Ustaše were only permitted to leave Italy and Italian-occupied territory in Yugoslavia after Mussolini extracted a written confirmation of the 1927 pledge,[7] allowing him to reach Zagreb in the early morning of 15 April with 195 Ustašas.

The decision to abandon armed resistance to the Axis powers so early placed the Yugoslav government-in-exile in a poor position, further weakened by quarreling ministers who appeared united only in their opposition to communism.

[10] With defeat imminent, the Communist Party of Yugoslavia (Komunistička partija Jugoslavije, KPJ) instructed its 8,000 members to stockpile weapons in anticipation of armed resistance[11] (which spread throughout the country, except for Macedonia, by the end of 1941).

[12] Building on its experience in nationwide clandestine operation, the KPJ organised the Partisans[13] into a resistance force led by Josip Broz Tito.

[14] The KPJ concluded that the German invasion of the Soviet Union had created favourable conditions for an uprising, and its politburo established the Supreme Headquarters of the National Liberation Army of Yugoslavia (Narodonooslobodilačka vojska Jugoslavije, with Tito its commander-in-chief) on 27 June 1941.

[17] The Serb population living in the NDH was eager to join the Partisan struggle, due to their severe persecution by the Ustaše regime.

[20] A major contributor to Croat Partisan recruitment was the transfer of a large portion of the Adriatic coast to Italy through the Treaties of Rome as fulfilment of Pavelić's promise to Mussolini.

[23] On 26 and 27 November,[24] the pan-Yugoslav Anti-Fascist Council for the National Liberation of Yugoslavia (Antifašističko vijeće narodnog oslobođenja Jugoslavije, AVNOJ) was established in Bihać at the urging of Tito and the KPJ.

[27] During the first session of the AVNOJ, Tito tasked KPH central committee member Pavle Gregorić with setting up a supreme political body of the national liberation movement in Croatia as soon as possible.

[30] Balen, a former HSS member who was persuaded to join the KPJ by KPH central-committee secretary Andrija Hebrang, later headed the ZAVNOH propaganda department.

[30] After the successful spring 1943 offensive and recapture of most of the Banija, Kordun, and Lika regions by the 1st Corps, the ZAVNOH first convened in Otočac and Plitvice as Croatia's supreme representative political body on 13–14 June.

It adopted the Plivice Resolution, detailing the history of the Croatian people and their struggle for freedom, the backward nature of the Kingdom of Yugoslavia, the reign of terror of the NDH and the Chetniks, and the betrayal of the royal government in exile.

The resolution called for the recovery of Croatian lands seized by foreigners and "full and true democratic freedom and equality of Croats and Serbs".

[33] Between the ZAVNOH's first and the second sessions, the national liberation movement in Croatia grew from 25,000 to 100,000 fighters and increased its control – particularly on the coast, after the surrender of Italy.

On 20 September 1943, the ZAVNOH executive committee decided to add Istria, Rijeka, Zadar and other Croatian lands previously annexed by Italy to Croatia[34] (and thus to Yugoslavia).

[35] Tito criticised the ZAVNOH for assuming sovereignty in place of Yugoslavia, seeing the decision as an example of latent nationalism in the KPH leadership[36] (which controlled about fifty percent of the Yugoslav Partisan forces at the time).

Although some Partisan fighters resented the acceptance of HSS members, the official KPH position was that the newcomers were welcome and free to maintain their political views.

[43] He urged the party to ensure that the Partisan struggle was not perceived as exclusively communist, condemning "fanatics flying only the red flag" and extremism in the KPJ.

[46] Despite Pribičević's assertion that Croatian Serbs would pursue Croatia's interests in Yugoslavia, there was some resentment of their actual, perceived or expected position.

Although Hebrang insisted on teaching the Cyrillic script in all schools, he also said that Croatia Serbs had to accept their a minority status (albeit with equal rights) in a Croatian state.

The Allied forces in Yugoslavia were represented by Red Army Colonels Vladimir Goroshchenko and Mikhail Bodrov, British Major Owen Reed, and US Office of Strategic Services Captain George Selvig.

In September 1944, Tito criticised the ZAVNOH regulation introducing religion as a mandatory educational subject in Croatia's Partisan-held territory.

[63] Twenty five years after the war, during the 1971 political upheaval known as the Croatian Spring, the then-leaders of the League of Communists of Croatia (Savez komunista Hrvatske, SKH) publicly stated that the status of Croats in Yugoslavia had not been resolved in accordance with ZAVNOH decisions, and they advocated a reform of the Yugoslav federation.

[67] Although the SKH leadership was forced to resign by Tito and many of their policies were reversed, their efforts to reform Yugoslavia were considered by a federal commission in 1971.

[73] In January 1992, the Badinter Commission ruled that the Serbs living in Croatia and Bosnia and Herzegovina were entitled to the rights attributed to minorities and ethnic groups under international law; however, it did not use the term "constituent nation".