The Kingdom of Italy (Italian: Regno d'Italia) was a state forged in 1861 by the unification of Italy under the influence of the Kingdom of Sardinia which is its legal predecessor State. It existed until 1946 when the Italians opted for a republican constitution.
Italy declared war on Austria in alliance with Prussia in 1866: despite an unsuccessful campaign, it received the region of Venice following Bismarck's victory. Italian troops entered Rome in 1870, ending more than one thousand years of Papal temporal power. Italy accepted Bismarck's proposal to enter in a Triple Alliance with Germany and Austria 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. So, in 1915, Italy accepted the British invitation to join the Allies in World War I because the western allies promised territorial compensation (at the expense of Austria-Hungary) for participation that were more generous than Vienna's offer in exchange for Italian neutrality. Victory in the war gave Italy the status of a major power, with a permanent seat in the Council of the League of Nations.
During the time of the regime of the National Fascist Party from 1922 to its ousting in 1943, under the dictatorship of Benito Mussolini, the name often given by some historians to the Kingdom of Italy during this period is Fascist Italy. Under fascism, the Kingdom allied with Nazi Germany in World War II until 1943. In the remaining two years of World War II, the Kingdom of Italy switched sides to the Allies after ousting Mussolini as Prime Minister and banned the Fascist party. The remnant fascist state that continued fighting against the Allies was a puppet state of Nazi Germany, the "Italian Social Republic", still led by Mussolini and his loyalist Fascists in northern Italy. Shortly after the war, civil discontent led to the Italian constitutional referendum, 1946 on whether Italy would remain a monarchy or become a republic. Italians decided to abandon the monarchy and form the Italian Republic, which is the present form of Italy today.