THE GERMAN CAMPAIGN IN THE BALKANS (SPRING 1941): PART II
http://www.army.mil/cmh-pg/books/wwii/balkan/20_260_2.htm

"III. German Propaganda

German propaganda efforts naturally took full advantage of this open rift between Serbs and Croats. The constantly repeated official line was that Germany and Italy desired the creation of an independent state of Croatia and that the military operations were being conducted only against the Serbs. However, when Hitler was first told of the open animosity among the various ethnic factions in Yugoslavia, he is said to have remarked: "That is none of our business. If they want to bash each others' heads in, let them go ahead."

"In a letter Mussolini wrote to Hitler on 29 December 1941, the former stated with reference to Yugoslavia:

Before next spring every nucleus of insurrection must be wiped out or else we run the risk of having to fight a subsidiary war in the Balkans. The first territory to be pacified is Bosnia, then Serbia and Montenegro. The military operations must be conducted with great determination and must lead to a real and complete disarmament of the population, this being the sole guarantee I for avoiding surprise in the future. For this purpose our military forces must cooperate according to a common plan to prevent duplication of effort and achieve the desired result with a minimum of manpower and materiel."