In the medieval age kings had the power to influence the election of the Popes and bishops. -> http://en.wikipedia.org/wiki/Investiture_Controversy
Many Popes had children and led their own armys. Maybe no Pope had a really big one but they could have declared every enemy leader to be antichrist / excommunicate them.
I can understand that no king would have dared to attak a Pope as they were belived to be God's deputys on earth. But why did no Pope exploit his powers? Especially after the Investiture Controversy was settled and they weren't this easy to influence by the european kings
We are discussing hundreds of years of history here, so I will have to address the issue in general terms, but i'll try to use some specific examples as well.
The military history of the Papal States--the nation in the central part of the Italian peninsula--is checkered at best. There were precious few crushing victories, and more than a handful of humiliating defeats. Simply put, the Papal States was never more than a regional power and could hardly challenge the large powers of Europe at any point. Many times, the nation struggled to project power at any distance beyond its borders. The big players of Western Europe (France, the Holy Roman Empire, Spain, England, and others) had little to fear from a hypothetical Papal invasion.
If we examine another aspect of papal power we could look at the idea of excommunicating leaders that came into conflict with the pope. Most famous excommunications of national leaders have not historically led to a positive result for the papacy. Gregory VII got Henry IV of the HRE to come to heel in 1077, but ended up losing out on the long run. It went worse for Pope John XXII vs. Louis IV of Bavaria in 1324, with the pope dying in exile and an antipope being elected and Louis being in control of Rome. Pius V vs. Queen Elizabeth I of England in 1570 resulted not in a chastised monarch, but rather in a schism. Little effect was felt when Napoleon was excommunicated either. Formal excommunication is a sledgehammer move, and since the papacy rarely if ever had the power to enforce its will it was seldom effective in achieving the stated aims.
He was speaking about modern popes and not medieval ones, but Jose M. Sanchez has some insights into the papacy that still apply here. For all the power that the pope certainly has,
two limitations of knowledge and influence weigh heavily on any decision the pope makes: he must ask himself is his sources are reliable and he must act with the constant apprehension that his words will not be heeded; or worse, that he will be disobeyed and the disobedience itself may lead to schism
He also asserts that the pope
has power and influence only over believers; and believers can quickly become unbelievers if pressed to do something to which they object
So the pope can push on an issue, but only as far as Catholics are willing to be pushed. If these Catholics identify more closely as subjects of a particular nation than they do as subjects of the pope, then getting them to act against their own identified "home" would be difficult at best and would risk schism at worst. Any time the pontiff pushed further than his followers were willing to go the limitations of the pope's power were demonstrated, and since the pope had more "theoretical" power than "real" power that was something to be avoided.
So, both the pope's military power and theological power had real limits, and at no point was either type of power sufficient to run roughshod over the whole of Europe.
The Pope is actually already considered to be a Monarch. It's just that his position comes with being elected, making the Pope one of the few elected Monarchs left on the planet.
'But why did no Pope exploit his powers?'
Oh, they surely did exploit their powers throughout the ages.