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.

Background

In 1915, the Kingdom of Italy entered the First World War on the side of the Entente following the signing of the Treaty of London, which promised Italy territorial gains at the expense of Austria-Hungary. The treaty was opposed by representatives of the South Slavs living in Austria-Hungary, who were organised as the Yugoslav Committee. Following the 3 November 1918 Armistice of Villa Giusti, the Austro-Hungarian surrender, and well ahead of the Paris Peace Conference, Italian troops moved to occupy parts of the eastern Adriatic shore that had been promised to Italy under the Treaty of London. To counter Italian demands, the internationally unrecognised State of Slovenes, Croats and Serbs, a proto-state that was carved from areas of Austria-Hungary populated by the South Slavs, authorised the Yugoslav Committee to represent it abroad. The short-lived state, which would soon seek union with the Kingdom of Serbia to establish the Kingdom of Serbs, Croats and Slovenes, laid a competing claim to the eastern Adriatic coast and islands. This claim was supported with the deployment of the Royal Serbian Army to the area. Major powers did not recognise the new unified state before June 1919.

The Treaty of London's provisions were a major point of dispute between Italy and the remaining Entente powers at the Paris Peace Conference. The chief Italian representatives, Prime Minister Vittorio Emanuele Orlando and foreign minister Sidney Sonnino demanded enforcement of the treaty arguing that possession of the promised territories was vital for Italian security, and called for the annexation of the city of Rijeka () based on the principle of self-determination. Orlando was prepared to abandon claims in Dalmatia, except Zadar () and Šibenik (), while insisting on annexing Rijeka. Sonnino held a view that was summarised as "Pact of London plus Fiume", portraying the claim as a matter of Italian national honour.

The British and French governments would not publicly endorse any claims exceeding those the treaty afforded, while privately believing Italy deserved little due to its reserved attitude towards Germany in the early stages of the war. Regarding Dalmatia, the British Prime Minister David Lloyd George supported a free-city status for Zadar and Šibenik only, while the French Prime Minister Georges Clemenceau supported such a status for Zadar only. Clemenceau and Lloyd George were content for the President of the United States Woodrow Wilson to hold in check Italian ambitions in the Adriatic by advocating self-determination for the area under point nine of his Fourteen Points.

Wilson deemed the Treaty of London a symbol of the perfidy of European diplomacy and held it invalid by dint of the legal doctrine of clausula rebus sic stantibus – that the treaty was no longer applicable because there had been fundamental changes in the circumstances in which it was negotiated, namely the dissolution of Austria-Hungary.

Zones of occupation

The Naval Commission for the Adriatic was established based on the decision of the Allied Naval Council of the Supreme War Council on 5 November 1918. The commission, composed of admirals delegated by the four powers, was tasked with coordinating the Allied occupation forces. It was also intended to maintain order, normalise civilian life, manage maritime transport, oversee the distribution of the Austro-Hungarian fleet, and deal with other related issues that the Paris Peace Conference had permanently settled. The commission was first convened in Rijeka. Later, it moved to Venice and Rome. Initially, it was chaired by the Italian Rear Admiral . He was replaced by Admiral Paolo Thaon di Revel based on a decision of the Allied Naval Council in Paris on the same day.

The commission agreed on zones of occupation of the eastern Adriatic shores. The occupation plan was never fully enforced; only Italy deployed a large force to the area. The assignment of the Italian zone was the result of a fait accompli. Italy interpreted the armistice terms as allowing them to establish the armistice line per the claims laid out in the Treaty of London. Italy had already established a firm foothold in the claimed area by the time the zones were defined in Venice on 16 November, in a meeting of the commission chaired by Revel. Other sources say the zones were defined on 26 November at a commission meeting in Rome. Italy would control northern parts of Dalmatia corresponding to the Dalmatian territory promised under the Treaty of London. The US would control the southern Dalmatian coast, and France would control the coasts of the southernmost part of Dalmatia, the Kingdom of Montenegro and the further south coast of the Principality of Albania. The United Kingdom was to control the Kvarner Gulf.

The former Austro-Hungarian realm of the Kingdom of Dalmatia encompassed the entire Italian and American zones of occupation and a portion of the French zone. In the territory of the Kingdom of Dalmatia as a whole, Croats comprised 80%, Serbs comprised 17%, and Italians comprised 3% of the population. According to the 1910 Austro-Hungarian census, all Dalmatian municipalities had a Slavic majority of 95% or more, except Kotor () and Zadar. In the former, the Slavic population accounted for 80% of the total, Italians for 1%, German speakers comprised 3% and foreign nationals accounted for more than 11% of the population. The city of Zadar had an Italian majority. Rijeka was ethnically mixed and both sides in the dispute claimed majority in Rijeka, but defined the city limits differently. Italians argued that the city consisted of the former Austro-Hungarian corpus separatum territory only, but the South Slavs argued that the suburb of Sušak should also be accounted for.

British zone

End of Austro-Hungarian rule

In late October 1918, the governing organ of the State of Slovenes, Croats and Serbs, the Zagreb-based National Council of Slovenes, Croats and Serbs, appointed Rikard Lenac to take over the administration of Rijeka on behalf of the newly proclaimed state. The move was coordinated with the administratively separate suburb of Sušak. The takeover of authority occurred on 29 or 31 October, according to varying sources. Military authority in Rijeka and Sušak was assumed by Lieutenant Colonel Petar Teslić, who commanded eight battalions of the 79th Infantry Regiment of the former Austro-Hungarian Common Army normally based in Otočac, and pro-Yugoslav National Guard volunteers, largely consisting of high-school pupils. Leading ethnic Italians living in the city established the Italian National Council of Fiume () and declared the desire to incorporate the city into Italy. They dispatched a delegation to Revel in Venice to request aid. Lenac asked Zagreb to send additional troops as well. There were clashes between Italian and South Slavic communities in the city, each side claiming the city based on the self-determination principle. While Italy said there was an Italian majority in the city within the boundaries of the former Austro-Hungarian , the South Slavs said the entire city, including the suburb of Sušak, had a South Slavic majority.

Occupation of Rijeka

On 2 November 1918, a group of United States Navy ships sailed into the Port of Rijeka. The next day, they were followed by a French and a British force. Colonel Sydney Capel Peck led the British mission to Rijeka. On the same day, a joint allied command was established in Rijeka, ostensibly to prevent further ethnic violence. On 3 November, the day the armistice was signed, Italian armed forces gained control of much of the nearby Istrian peninsula, including the cities of Trieste, Rovinj () and Pula (). The Italian navy arrived in the Port of Rijeka on 4 November; the first group consisted of the battleship Emanuele Filiberto; and the destroyers Francesco Stocco, Vincenzo Giordano Orsini, and Giuseppe Sirtori. On 5 November, the French destroyers Touareg and Sakalave brought further reinforcements.