Pope Julius II (reign 1503-1513) was nick-named the 'Warrior Pope' for his incessent wars, first the Venetians and later France. As with most Papal elections during this time, his succession was largely secured through bribery. He was, however, known for his aggression as a Cardinal. As his rivalry with Rodigro Borgias grew, he persuaded Charles VIII to invade Rome in 1494. In 1506 he also founded the famous Swiss Guards, which essentially acted as the Pope's private army. As we all know, this legacy is still visible in Vatican today.

(EDIT) Source: Shaw, Christine, Julius II: The Warrior Pope, 1993

Well there's always Julius II, the guy who commissioned the ceiling of the Sistine Chapel and whose nickname actually was the "Warrior Pope." Julius II's papacy, apart from the art that he patronized, was mostly spent trying to push the French out of Italy, which he largely succeeded at. Julius was at war pretty much all the time and was known for wearing his armor underneath his robes. He was also quite vulgar and was known as being a womanizer before becoming pope (he had three daughters while a cardinal). He also famously declared before going into battle against the French that his troops would soon see who had the bigger testicles, him, or the king of France

Clement VII would be perfect, except for the fact that he's an Anti-Pope in the eyes of the modern church. What that means, basically, is that there were two popes. To explain this to the best of my very-non-expert understanding, One was supported by the French and their allies and lived in Avignon in France, one lived in Rome, and was given lip service by a lot of nations with grudges against France. Eventually the Roman line won out, so the Avignon popes are Anti-Popes.

Anyways, Clement VII was born as Robert of Geneva, and he was a bulldog. It's generally agreed that he personally commanded troops in battle as a bishop and cardinal (he was made a cardinal before the age of 30, the go-getter) and ordered a mass-execution of civilians at Cesena in 1377. A year later, the French faction annointed him as pope (while in his mid-thirties). Definitely had something to do with his warrior's reputation. Incidentally, there's some propaganda floating around that he was capable of decapitating a man with pike, which certainly makes for an interesting image.

Source: http://www.newadvent.org/cathen/13096c.htm

As a related question, how did Catholics feel about taking up arms against a pope? I know there were situations like the Western Schism where people recognized different popes. I'm curious about people who recognized a man as the legitimate supreme religious authority, but still took the battlefield against him.

Did rank and file troops have a different viewpoint than the nobility and military commanders? Did people make precautions to prevent accidentally killing the pope?

Yes! Warrior Pope Leo IX, Pope from 1049 to 1054, lead an army against the normans, was defeated and captured at the Battle of Civitat in 1053. Leo had apparently been begged by local lords, like William of Auplia who feared the growing Norman presence in the region, to attack the Normans. He seems to have been treated rather well by the Normans but was only set free after he acknowledge Robert Gusicards conquests of Calabria and Apullia. The southern Normans have quite a history of conflict and alliances with the papacy. If you want to read more about the battle it's all in G.Loud's book on Robert Guiscard.

Did the Vatican ever appoint people as "Paladins"? Is that an actual word/class that used to exist? People who were said to sway the battlefield in some way through their faith? Does that word have any connotation outside of the fantasy genre that co-ops it?

All of this answers are fascinating. Could anyone recommend a good text on the history of Catholic Church and Papal politics?

Obviously, without any religious bias or hiding any if the dirt away.