The offensive began with the battle of Malka Nidzhe on 12 September 1916. After two days of fighting the Serbians achieved a breakthrough around Gornichevo and forced the Bulgarian 8th division to retreat. Meanwhile, the battle for Kajmakcalan also began. The fall of Florina on 23 September and the retreat of the Bulgarian First Army forced some changes in the Central Powers command structure. On 27 September general Kliment Boyadzhiev was replaced as commander of the First Army by general Dimitar Geshov. The Army itself exchanged headquarters with the Eleventh Army, now under the command of general Arnold von Winckler, and came under the new Army Group "Below", replacing Army Group "Mackensen" which was dissolved on the Macedonian Front on 30 July 1916 as field marshal Mackensen was needed in northern Bulgaria to take command of the forces concentrating for the operations against Romania. The former units of the First Army, now under German command, continued fighting around Monastir till December when the Allied offensive was finally halted.