'Ostern' (i) was a German and Croat operation against the partisan forces of Marshal Josip Broz in the puppet state of Croatia within Axis-occupied Yugoslavia (15/17 April 1943).
The undertaking’s objective was the engagement and destruction of a 400/500-strong group of the 'Banija' Partisan Detachment, which was derailing and attacking trains and ambushing road traffic, in the area of Sunja 12.5 miles (20 km) to the south-south-east of Sisak. The German contribution to the operation was elements of several formations and units including Generalleutnant Karl Eglseer’s 114th Jägerdivision, Generalleutnant Josef Brauner von Haydringen’s 187th Reserve-Division, the 823rd Landesschützenbataillon, the 924th Landesschützenbataillon, the 925th Landesschützenbataillon, and the 206th Panzerzüg and 209th Panzerzüg armoured trains, while that of the Croats was elements of Pukovnik Arnold Redelstein’s 2nd Mountain Brigade, 2/5th Mountain Brigade, one company of the 17th Ustase Battalion, and one Ustase company from Čaprag.
The partisans had left the area before the start of the operation, which therefore yielded no useful results. There were no losses on either side.