During World War II, the Croatian ultra-nationalist Ustaa persecuted nearly two million Serbs, Jews, and Roma in the Independent State of Croatia (present-day Croatia and Bosnia). Political analysts often cite this genocide as proof that ethnic violence within the region is inevitable. However, an equally important reality is that within just four years, Ustaa excesses had provoked a widespread popular reaction against the violence and the national exclusivity that inspired it. Although many people in Croatia and Bosnia initially celebrated the collapse of Yugoslavia in 1941 and supported the declaration of Croatian independence, the Ustaa's brutal treatment of minority groups quickly alienated much of the population. Opposition to ethnic persecution took many forms, including assisting people targeted by the government, hiding victims or helping them to escape from the country, aiding political prisoners, and, publicly protesting discriminatory measures. Within the concentration camps, prisoners of different ethnic backgrounds came together in food sharing and news-gathering cooperatives in a common effort to survive.
This rejection of ethnic violence served to discredit the extreme Croatian nationalism represented by the Ustaa - and also its Serbian counterpart. The result was a resurgence of Yugoslavism, a renewed emphasis on the interdependence of Serbs, Croats, Bosnian Muslims, and others.