Vatican City is an independent state established by treaty with Italy in 1929. From 752 to 1870 AD, the realm of the Holy See included Rome and surrounding areas. After Prussia defeated France in 1870, the Papal States lost its protector. Italian troops marched into Rome in September and made it the capital of their recently unified nation. Pope Pius IX retreated behind the walls of Vatican City, only months after he declared his office to be infallible during the First Vatican Council.
With an area of 44 hectares, Vatican City is the smallest country on earth.