The Treaty of Rome, signed in January 1924, served to formalize the annexation of Fiume, which had been a contentious issue following the end of World War I. The city had declared itself an independent state under the leadership of Italian nationalist Gabriele D'Annunzio in 1919, leading to a brief but high-profile occupation. This self-proclaimed state was characterized by a strong sense of nationalism and Italian identity but encountered opposition from the local Slavic population. The Treaty of Rapallo in November 1920 initially aimed to mediate the claims over Fiume, but the situation remained fraught until Italy and the Kingdom of Serbs, Croats and Slovenes (later Yugoslavia) negotiated the Treaty of Rome.

In March 1924, the agreement came into full effect, enabling Italy to assert its control over Fiume officially. This annexation was not just a matter of shifting territory; it represented deeper ideological shifts in the region, reflective of rising nationalism and the struggles to define national identity in the tumultuous interwar years. Fiume's incorporation symbolized the reassertion of Italian cultural and historical claims to the city, which had a significant Italian-speaking population, thereby justifying Italy's claim on ethnic grounds.

The annexation was met with mixed reactions, as it prompted further national tensions within the region. The significant Slavic population in Fiume worried about their rights and existence under Italian rule. Many viewed this as part of a broader narrative of Italian expansionism and the push for a greater Italian state, themes that would resurface in the lead-up to World War II. Fiume's aspiration for autonomy had failed to materialize, marking a pivotal moment in Italian nationalism's trajectory between the wars.