Italy declared war on Austria in alliance with Prussia in 1866 and received the region of Veneto following their victory. Italian troops entered Rome in 1870, thereby ending more than one thousand years of Papal temporal power. Italy entered into a Triple Alliance with Germany and Austria-Hungary in 1882, following strong disagreements with France about the respective colonial expansions. However, even if relations with Berlin became very friendly, the alliance with Vienna remained purely formal as the Italians were keen to acquire Trentino and Trieste, corners of Austria-Hungary populated by Italians. So in 1915, Italy accepted the British invitation to join the Allied Powers, as the western powers promised territorial compensation (at the expense of Austria-Hungary) for participation that was more generous than Vienna's offer in exchange for Italian neutrality. Victory in the war gave Italy a permanent seat in the Council of the League of Nations.
"Fascist Italy" is the era of National Fascist Party government from 1922 to 1943 with Benito Mussolini as head of government. The fascists imposed totalitarian rule and crushed the political and intellectual opposition, while promoting economic modernization, traditional social values and a rapprochement with the Roman Catholic Church. According to Payne (1996), " Fascist government passed through several relatively distinct phases". The first phase (1923–1925) was nominally a continuation of the parliamentary system, albeit with a "legally-organized executive dictatorship". Then came the second phase, "the construction of the Fascist dictatorship proper, from 1925 to 1929". The third phase, with less activism, was 1929 to 1934. The fourth phase, 1935–1940, was characterized by an aggressive foreign policy: war against Ethiopia, which was launched from Eritrea and Italian Somaliland; confrontations with the League of Nations, leading to sanctions; growing economic autarky; and the signing of the Pact of Steel. The war itself (1940–1943) was the fifth phase with its disasters and defeats, while the rump Salò Government under German control was the final stage (1943–1945).
Italy was an important member of the Axis powers in World War II, until it switched sides to the Allies in September 1943 after ousting Mussolini and shutting down the Fascist Party in areas (south of Rome) controlled by the Allied invaders. The remnant fascist state in northern Italy that continued fighting against the Allies was a puppet state of Germany, the Italian Social Republic, still led by Mussolini and his Fascist loyalists. Shortly after the war, civil discontent led to the constitutional referendum of 1946 on whether Italy would remain a monarchy or become a republic. Italians decided to abandon the monarchy and form the Italian Republic, the present-day Italian state.
The Kingdom of Italy claimed all of the territory which covers present-day Italy and even more. The development of the Kingdom's territory progressed under Italian re-unification until 1870. The state for a long period of time did not include Trieste or Trentino-Alto Adige/Südtirol, which are Italian territories today and only annexed them in 1919. The Triple Entente promised to grant to Italy – if the state joined the Allies in World War I – several territories including former Austrian Littoral, western parts of former Duchy of Carniola, Northern Dalmazia and notably Zara, Sebenico and most of the Dalmatian islands (except Krk and Rab), according to the secret London Pact of 1915.
After the compromise was nullified under pressure of President Woodrow Wilson with the Treaty of Versailles, Italian claims on Northern Dalmazia were voided. During World War II, the Kingdom gained additional territory: it gained Corsica, Nizza and Savoia from France after its surrender in 1940, territory in Slovenia and Dalmazia from Yugoslavia after its breakup in 1941 and Monaco in 1942. After World War II, the borders of present-day Italy were founded and the Kingdom abandoned its land claims.