'Kastanie' was a German and Croat operation against the partisan forces of Marshal Josip Broz Tito in the puppet state of Croatia of German-occupied Yugoslavia (11/12 May 1944).
The undertaking’s objective was the destruction of three battalions of the 4th Assault Division, which was believed to be concentrating in the area of Prijedor, Sanski Most and Omarska to the east of the Sana river in western Bosnia. The German element of the Axis force comprised the 92nd Grenadierregiment (mot.), 1/383rd and 2/383rd Grenadierregiment of Generalleutnant Emil Zellner’s 373rd Division (kroatisch), and the Panzergrenadiersturmbataillon of the 2nd Panzerarmee, and that of the Croats was the 2nd Battery and 3rd Battery/13th Artillery Group.
The operation was launched immediately after the end of 'Reinhard', and the Axis forces swept the planned area, and became involved in a modest number of firefights whenever they encountered partisan groups. The partisan main force avoided contact, however, and temporarily moved out of the area. The Germans reported no Axis casualties, and claimed 108 partisans killed and 134 captured.