The uprising started with the attack of four rebel detachments on the Drvar garrison of the Croatian army, which consisted of 400 Ustaše and Home Guard soldiers, early on 27 July.

The communists gradually forged their Partisan republic, and on 30 July, they established the Military-Revolutionary Council, which became the supreme governing institution for the entire region.

During the next two months, rebels managed to capture additional territory including Mrkonjić Grad, Kulen Vakuf and many other Western Bosnian towns.

Based on agreement with non-communist rebel leaders, the Second Italian Army peacefully took control over the region and temporarily shielded local Serbs from the Independent State of Croatia on 26 September 1941.

To fight against Chetniks who gained substantial support of the local population, the communists established two Anti-Chetnik Battalions in March and April 1942 and reestablished their control over the region of Drvar at the beginning of July 1942.

[3] In the weeks following the invasion, the Communist Party of Yugoslavia (Serbo-Croatian Latin: Komunistička partija Jugoslavije, KPJ) managed to tie up its cells.

Some members of the KPJ and the Union of Communist Youth of Yugoslavia (Serbo-Croatian Latin: Savez komunističke omladine Jugoslavije, SKOJ) were returnees from the defeated Royal Yugoslav Army, including Zdravko Čelar, Slavko Rodić, and Rajko Bosnić.

[4] KPJ committees in Drvar and Bosansko Grahovo succeeded in obtaining rifles, light machine guns, and ammunition belonging to the surrendered Royal Yugoslav Army, and hid them from German, Italian and later Ustashe searches.

[10] On 17 July, communists set up the Headquarters of Guerrilla Detachments for Bosanski Petrovac with Zdravko Čelar in command, Vaso Kelečević as his deputy and Ilija Došen as the political commissar.

[11] Between 20 and 26 July, the first detachment of fifty men was formed in the village of Kamenica (commander Jole Marić, political commissar Nikola Kotla).

[12] The Ustaše genocidal policy resulted in a mass uprising of Serbs which began on 27 July 1941 in Bosnian Krajina (Western Bosnia) and Lika simultaneously.

[13][non-primary source needed] Since 22 June 1941, more than 2 months after Axis invasion of Yugoslavia, the communists also planned to organize an uprising in Bosnian Krajina on 1 August 1941.

[16][attribution needed] Only in September, after the Stolice conference, would communist rebels adopt the name "partisans", and nationalists around same time begun calling themselves "chetniks".

[24] The Italian Military Intelligence Service reported that the uprising began on 26 July 1941 when rebels attacked isolated gendarmerie stations, railroads and communication lines.

[26][non-primary source needed] On 26 July 1941, in a road ambush near village Pasjak, a group of Serb nationalists, including Nikica Kecman and three of his men, killed Croatian major Ferdinand Konrad who was travelling from Drvar to Prijedor by car.

[27][28][non-primary source needed][18] The ambush was actually targeted at logornik Kuharski, who had issued an order to kill 40 notable Serbs from Drvar, but was still considered a success as it ignited the uprisings in Lika and Krajina.

Thus, their Headquarter of Guerrilla Detachments for Bosnian Grahovo and Surrounding (Serbian: Штаб герилских одреда за Босанско Грахово и околину) decided to start the uprising early the following day, on 27 July 1941.

][40][non-primary source needed] In Drvar, Ustaša Mijo Sarić fired heavy machine guns at rebels from the tower of local church.

[41][non-primary source needed] The rebels captured all surviving enemy soldiers, including 120 members of Croatian Home Guard, as well as a substantial quantity of arms and ammunition.

[52] Since the first days of the uprising in Bosnian Krajina, the communists gradually forged their Partisan republic, and on 30 July 1941, established a Military-Revolutionary Council that became the supreme governing institution for the whole region.

[56] As the uprising spread from Drvar to other parts of Western Bosnia, the Serb rebels in Banja Luka, Jajce, Prijedor, Livno, Dubica and Sisak were actually supported by Italy, both in politics and in arms.

The Chetnik leaders wished to save Serbs from their most feared and violent enemy - Ustaše, even if they had temporarily to accept the rule of Italians and Germans.

[62] After Italians captured the territory previously held by the rebels, the Chetniks gained the confidence of the local population, especially in central Bosnia and the region of Bosansko Grahovo.

[69] The pressure of the Grmeč Anti-Chetnik Battalion forced Uroš Drenović to sign an accord with representatives of the Independent State of Croatia on 25 April 1942.