When Emperor Constantine left Rome, in A.D. 330, to establish the new Roman capital, Constantinople (now called Istanbul, in Turkey), the Papacy was left alone in the city of Rome.
To establish its authority over kings, it forged documents, the «False Decretals», among which was the «Donation of Constantine». This document aimed at proving that Constantine had officially bequeathed the city of Rome to Pope Sylvester I.
This is how the Papacy «seized the kingdom by intrigue».