Italian unification was the political and social movement that annexed different states of the Italian peninsula into the single state of Italy in the 19th century. There is a lack of consensus on the exact dates for the beginning and the end of this period, but many scholars agree that the process began with the end of Napoleonic rule and the Congress of Vienna in 1815, and approximately ended with the Franco-Prussian War in 1871, though the last città irredente did not join the Kingdom of Italy until after World War I.