In September 1941, Partisans organized sabotage at the General Post Office in Zagreb.As the levels of resistance to its occupation grew, the Axis Powers responded with numerous minor offensives. There were also seven major Axis operations specifically aimed at eliminating all or most Yugoslav Partisan resistance. These major offensives were typically combined efforts by the German Wehrmacht and SS, Italy, Chetniks, the Independent State of Croatia, the Serbian collaborationist government, Bulgaria, and Hungary. The First Anti-Partisan Offensive was the attack conducted by the Axis in autumn of 1941 against the "Republic of Užice", a liberated territory the Partisans established in western Serbia. In November 1941, German troops attacked and reoccupied this territory, with the majority of Partisan forces escaping towards Bosnia. It was during this offensive that tenuous collaboration between the Partisans and the royalist Chetnik movement broke down and turned into open hostility. After fruitless negotiations, the Chetnik leader, General Mihailović, turned against the Partisans as his main enemy. According to him, the reason was humanitarian: the prevention of German reprisals against Serbs. This however, did not stop the activities of the Partisan resistance, and Chetnik units attacked the Partisans in November 1941, while increasingly receiving supplies and cooperating with the Germans and Italians in this. The British liaison to Mihailović advised London to stop supplying the Chetniks after the Užice attack , but Britain continued to do so. On 22 December 1941 the Partisans formed the 1st Proletarian Assault Brigade  - the first regular Partisan military unit capable of operating outside its local area. 22 December became the "Day of the Yugoslav People's Army".. When was the first "Day of the Yugoslav People's Army?"