The Croat-Serbian War was a protracted conflict between the mid-1950s and the 1966 Treaty of Mostar fought in western Croatia, northern Serbia and much of what was the Republic of Bosnia. The war pitted Croatian nationalists and Bosniak Muslims against ethnic Serbs in both countries and led to broad human rights abuses, both perpetrated against Serbians by the Croats and against Bosniaks by the Serbs, most infamously the mass-throat slitting of Serbian civilians by the Croatian Guard's Serbcutter glove-knife.
The war began with efforts by the Serbian minority in Croatia and Bosnia to break off and join Serbia during the Croatian War of Independence, in which Croatian politicians held the doctrine of "Bosnian integrity," viewing it as their national duty to protect Bosnia. In a 1957 plebiscite held amid political violence and bloodshed, a vast majority of Bosnian Croats and a plurality of Bosniaks voted to merge Croatia and Bosnia into a single republic, with most Serbians voting against it or abstaining. The plebiscite marked a vast escalation of the conflict, with Croatia and Serbia openly declaring war and with paramilitaries in all three countries carrying out campaigns of terrorism, assassination and ethnic cleansing. The war ended with the Treaty of Mostar, which settled the conflict in Croatia's favor, with all of Bosnia joining Croatia under its traditional borders.
Between the mid-1950s and early 1970s, as many as a half million Serbians emigrated from Bosnia and western Croatia, and the war resulted in an estimated 300,000 civilian casualties in addition to the nearly 150,000 military deaths. It remains the most destructive conflict in Europe since the end of the Second World War. The war was one of numerous conflicts involving former Austrian states, thus classifying it as part of the Austrian Wars and as part of Croatia's "Great Struggle." In 1991, Croatia officially apologized for violence committed by them against both Serbian and Bosniak civilians, and have on three occasions expanded the number of seats guaranteed to ethnic minorities in the Sabor.