Allied occupation of the eastern Adriatic

The occupation of the eastern Adriatic by the Allies of World War I was a military mission that followed the First World War and lasted from November 1918 to September 1921. Naval assets and troops of the United Kingdom, the Kingdom of Italy, France, and the United States were deployed to parts of the territory of former Austria-Hungary, especially the region of Dalmatia, the city of Rijeka, and coastal areas of the Kingdom of Montenegro. The occupation was intended to resolve the disposal of assets of the Austro-Hungarian Navy and the settlement of Italian territorial claims on the eastern coast of the Adriatic Sea. Those claims, largely corresponding to the award made under the Treaty of London used to entice Italy to enter the war on the side of the Allies, conflicted with the territorial claims of the nascent Kingdom of Serbs, Croats and Slovenes (later renamed Yugoslavia) and its predecessor states, as well as the principle of self-determination outlined in the Fourteen Points of the US President Woodrow Wilson.

The Allies divided the eastern Adriatic coast into four zones of occupation. The Italian zone was determined to correspond to the Treaty of London award and centred on Zadar and Šibenik in northern Dalmatia. Central Dalmatia and the city of Split were assigned to the United States. Southern Dalmatia, between Dubrovnik and Kotor, and the coast of Montenegro, became a French zone of occupation. The British were assigned to the Kvarner Gulf in the northern Adriatic, centred on Rijeka. Italy claimed the city based on the Treaty of London and on the principle of self-determination. Littoral areas outside the four zones of occupation were controlled either by Italian forces in Istria or by the Army of the Kingdom of Serbs, Croats and Slovenes (or the Royal Serbian Army before December 1918). All major ports had a military presence from all four allied nations.

The occupation was marked by Italian efforts to pursue territorial claims and conflicts with civilian populations and local authorities in some areas. Otherwise, the local population in Dalmatia generally welcomed the Allies. In ethnically mixed Rijeka, the reception reflected the ethnic composition. The British occupation of Rijeka was affected by the takeover of the city by Gabriele D'Annunzio in September 1919, shortly after Francesco Saverio Nitti replaced Vittorio Emanuele Orlando as the Italy's prime minister and the new government agreed with the British and French that Rijeka should be a city-state. To prevent the implementation of the agreement, D'Annunzio proclaimed the Italian Regency of Carnaro and the Allies retreated from the city. In the French zone, Italy and France came into conflict over Montenegro's future, as well as over support for opposing factions in the January 1919 Christmas Uprising in Montenegro.

The occupation was concluded following the transfer of the remaining Austro-Hungarian naval assets to Italy and the Kingdom of Serbs, Croats and Slovenes, and by the 1920 Treaty of Rapallo, which determined the border between the two countries. The treaty also established the Free State of Fiume in the Rijeka area, prompting D'Annunzio's removal from the city by the Regia Marina.