The Roman Catholic Pope is head of both the Roman Catholic Church: "Supreme Pontiff of the Universal Church", and a head of state: "Sovereign of the Vatican City State". Official list of Papal titles
At what point in history did the head of the Church become a temporal, as well as spiritual leader? What is the historical basis for this temporal power? What is its nature? Is the Pope effectively a monarch - King of the Vatican State? In matters of the Church, there is a 'Papal Doctrine of Infallibility' - does this also apply to the Pope's temporal role? Is he an absolute monarch?
In short: What was is the nature and history of the office "Sovereign of the Vatican City State" and its historical predecessors.
I realize there is undoubtedly a long and detailed history here, but an overview of the 'bullet points' would be nice - I have very little knowledge about this topic, which I find most intriguing.