The birth of the Kingdom of Italy was the result of efforts by Italian nationalists and monarchists loyal to the House of Savoy to establish a united kingdom encompassing the entire Italian Peninsula. In the context of the 1848 liberal revolutions that swept through Europe, an unsuccessful war was declared on Austria.

The Kingdom of Sardinia again attacked the Austrian Empire in the Second Italian War of Independence of 1859, with the aid of France, resulting in liberating Lombardy. In 1860–61, general Giuseppe Garibaldi led the drive for unification in Naples and Sicily, allowing the Sardinian government led by the Count of Cavour to declare a united Italian kingdom on 17 March 1861. In 1866, Victor Emmanuel II allied with Prussia during the Austro-Prussian War, waging the Third Italian War of Independence which allowed Italy to annexe the Venetia. Finally, as France during the disastrous Franco-Prussian War of 1870 abandoned its garrisons in Rome, the Italians rushed to fill the power gap by taking over the Papal States.

The Piedmontese Albertine Statute of 1848, extended to the whole Kingdom of Italy in 1861, provided for basic freedoms, but electoral laws excluded the non-propertied and uneducated classes from voting. The government of the new kingdom took place in a framework of parliamentary constitutional monarchy dominated by liberal forces. In 1913, male universal suffrage was adopted. As Northern Italy quickly industrialised, the South and rural areas of North remained underdeveloped and overpopulated, forcing millions of people to migrate abroad, while the Italian Socialist Party constantly increased in strength, challenging the traditional liberal and conservative establishment.

During the last two decades of the 19th century, Italy developed into a colonial power by forcing Somalia, Eritrea and later Libya and the Dodecanese under its rule. Italy, nominally allied with the German Empire and the Empire of Austria-Hungary in the Triple Alliance, in 1915 joined the Allies into the war with a promise of substantial territorial gains, that included western Inner Carniola, former Austrian Littoral, Dalmatia as well as parts of the Ottoman Empire.

The war was initially inconclusive, as the Italian army get struck in a long attrition war on the Dolomite mountains, making little progress and suffering very heavy losses. Eventually, in October 1918, the Italians launched a massive offensive, culminating in the victory of Vittorio Veneto. The Italian victory marked the end of the war on the Italian Front, secured the dissolution of the Austro-Hungarian Empire and was chiefly instrumental in ending the First World War less than two weeks later. During the war, more than 650,000 Italian soldiers and as many civilians died and the kingdom went on the brink of bankruptcy.

Under the Peace Treaties of Saint-Germain, Rapallo and Rome, Italy obtained most of the promised territories, but not Dalmatia (except Zara), allowing nationalists to define the victory as " mutilated". Moreover, Italy annexed the Hungarian harbour of Fiume, that was not part of territories promised at London but had been occupied after the end of the war by Gabriele D'Annunzio.

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