Yugoslavism, Yugoslavdom, or Yugoslav nationalism is an ideology supporting the notion that the South Slavs, namely the Bosniaks, Bulgarians, Croats, Macedonians, Montenegrins, Serbs, and Slovenes, belong to a single Yugoslav nation separated by diverging historical circumstances, forms of speech, and religious divides. During the interwar period, Yugoslavism became predominant in, and then the official ideology of, the Kingdom of Yugoslavia. There were two major forms of Yugoslavism in the period, the first of which is the regime-favoured integral Yugoslavism, promoting unitarism, centralisation, and unification of the country's ethnic groups into a single Yugoslav nation, by coercion if necessary. The approach was also applied to languages spoken in the Kingdom. The main alternative was federalist Yugoslavism, which advocated the autonomy of the historical lands in the form of a federation and gradual unification without outside pressure. Both agreed on the concept of National Oneness developed as an expression of the strategic alliance of South Slavs in Austria-Hungary in the early 20th century. The concept was meant as a notion that the South Slavs belong to a single "race", were of "one blood", and had shared language. It was considered neutral regarding the choice of centralism or federalism.
The Yugoslavist idea has roots in the 1830s Illyrian movement in Habsburg Croatia, where a group of intellectuals saw the unity of South Slavs within the Austrian Empire or outside of it as a protection against Germanisation and Magyarisation. Cooperative talks began with Serbian politicians and working to standardise Serbo-Croatian as a common language with orthographer Vuk Karadžić, with limited success. Following the Austro-Hungarian Compromise of 1867, the concept was rivalled by Trialism. Control of the Balkans by the Ottoman Empire and Austria-Hungary prevented practical implementation of Yugoslavist ideas until the Ottomans were pushed out of the Balkans in the 1912 First Balkan War and Austria-Hungary disintegrated in the final days of the First World War. During the war, preparations for unification began in the form of the Niš Declaration of Serbian war aims, the establishment of the Yugoslav Committee to represent South Slavs living in Austria-Hungary, and the adoption of the Corfu Declaration on principles of unification. The short-lived State of Slovenes, Croats and Serbs was proclaimed in the South Slavic lands formerly ruled by the Habsburgs at the end of the First World War. Its leadership primarily wanted unification with Serbia on a federal basis, while Serbia preferred a centralised state.
