Simultaneously , a multi-side civil war was waged between the Yugoslav communist Partisans , the Serbian royalist Chetniks , the Croatian fascist Ustashe and Home Guard , Serbian Volunteer Corps and State Guard , as well as Slovene Home Guard troops .

However , after 1941 , Chetniks extensively and systematically collaborated Collaboration with the Italian occupation forces Military_operation until the Italian capitulation , and thereon also with German and Ustashe forces Military_operation .

The Ustashe regime ( mostly Croats , but also Muslims and others ) committed Commitment genocide against Serbs , Jews , Roma and anti-Fascist Croats .