The thesis elaborates the period from the signing of the Treaty of London in 1915, the end of the World War I, the peace conference and negotiations to the signing of the Treaty of Rapallo in 1920. With the Treaty of London, Italy entered World War I on the side of the Entente, and in return it got promises about the eastern Adriatic coast. The plans of Italy and the Entente were revealed by a group of Croatian, Slovenian and Serbian political emigrants gathered in the Yugoslav Committee, which will then fight against the London Treaty and for the interests of the South Slavs of the Austro-Hungarian Monarchy. At the end of the war, Austria-Hungary collapsed, and the State of Slovenes, Croats and Serbs was proclaimed, which strived for unification with the Kingdom of Serbia and the Kingdom of Montenegro. The new state found itself in an unenviable situation without an effective military force and international recognition. During this time, the Italian army occupied large parts of the coast and imposed a hostile attitude towards the authorities of the State of Slovens, Croats and Serbs, and the Slavic population. This difficult situation accelerated the unification processes. After the proclamation of the Kingdom of Serbs, Croats and Slovenes, the diplomatic struggle began at peace conferences and negotiations, which in 1920 resulted with the Treaty of Rapallo, by which Goriška, Trieste, Istria and Zadar became Italian territory.