'Herbst I' was a German and Croat operation against the partisan forces of Josip Broz Tito in the puppet state of Croatia in German-occupied Yugoslavia (25/29 September 1943).
The objective was the destruction of the partisan forces (12th Division and 'Dilj' Partisan Detachment) in the Dilj mountain region within the area bounded by Nasice, Djakovo, Donji Andrijevci and Slavonski Brod.
The German contribution took the form of major elements of Generalleutnant Josef Brauner von Haydringenss 187th Reserve-Division, 939th Infanteriebataillon (kroatisches), and 6/Polizeiregiment 'Kroatien', while that of the Croats was the headquarters of Ustase Dopukovnik Antun Ilik’s 2nd Ustase Brigade with the 16th and 18th Ustase Battalions and half a battery of artillery, 6th Ustase Battalion, 2nd Bicycle Battalion, and three platoons of Croat military police from Slavonski Brod.
The partisans in the Dilj mountain region had been causing the Germans and Croats a great deal of trouble by sabotaging the main railway lines and derailing trains in South Slavonia. The operation was successfully concluded after five days of heavy fighting, and the Germans admitted Axis losses of 21 men killed, 69 wounded and 25 missing, while claiming 320 partisans counted dead plus an estimated 540 probably killed, and 710 estimated wounded.