During the Great War of 1914-1918, (World War I) both Italy and Serbia had received territorial promises of the allies Britain and France, once the war was over. France and Britain failed to meet their promises. Serbia then joined with neighboring Slavic ethnicity and thus formed a kingdom later known as Yugoslavia.
After the Great War, the Austro-Hungarian Empire was dissolved; then Italy occupied the “promised land”, to the northeast, which belonged to the Austro-Hungarian Empire. As a result, Italy came into conflict with Slavs, who mostly occupied these territories, known as the High Adriatic.
Fortunately, they reached an agreement that is based on the 1920 Treaty of Rapallo. Italy received much of the Austrian coast of the interior of Carniola and some areas of Carinthia.
Revolution, end of World War I and the Treaty of Rapallo
The main actor of the Treaty was the US President Woodrow Wilson. Not to be confused with a second Rapallo Treaty signed in 1922 between Russia and Germany, supposedly of cooperation and friendship!
During the Great War, Italy and Serbia had been given conflicting promises of territory by the Allies. When the War ended, Serbia united with other Slavic states to form the Kingdom of Serbs, Croats and Slovenes (or Yugoslavia) while Italy occupied the lands it had been promised, many of which had large Slavic populations. Disputes between the two countries, especially over the status of the city of Fiume, led to the 1920 Treaty of Rapallo.