In the IV century Constantine came to power. He permitted the practice of Christianity, and on the site of St. Peter’s burial he erected a basilica. Later the famous St. Paul’s Cathedral would be built here.
He moved the capital of the state to Byzantium, and organized the new state system. But it’s not about him now.
At the end of the V century, the Roman Empire disintegrates
Everything changed in 1870
After so many centuries, Italy is unified into a single state (a process beautifully called the "Risorgimento"). Victor Emmanuel II seizes Rome, and the then Pope Pius IX, having lost his power and influence, flees behind the walls of the Vatican. The "question of Rome" arises, which is not resolved for 60 years.
From 476 a period of feudal fragmentation begins. The rise of the papacy and of ecclesiastical power takes place.
By 756 an entire Papal Region had formed, with more than 3 million people at the best of times. The boundaries of the realm were difficult to draw: after all, the popes were constantly struggling to expand, annexing new lands. But it was not without losses either.
After so many centuries, Italy is unified into a single state. Victor Emmanuel II seizes Rome, and the then Pope Pius IX, having lost his power and influence, flees behind the walls of the Vatican. The "question of Rome" arises, which is not resolved for 60 years.