The Vatican’s history as the seat of the Catholic Church began with the construction of a basilica over St. Peter’s grave in Rome in the 4th century A.D.

The Pope used to rule the Papal States, which included most of Italy. Catholic popes tried to stop Italy from becoming one country because they feared to lose the control of at least one of the Papal States. In 1861, Italy was unified under the King of Savoy, but Rome and Latium remained unconquered. On September 20, 1870, Italian troops invaded and Rome became capital of the new kingdom. In 1929, Benito Mussolini decided to sign an agreement with the Holy See, called the Lateran Treaty, which established the Vatican State.