While today's Vatican is the center of The Catholic Faith, it was once the seat of power for what was known as The Papal States. These Territories (located mostly in Italy's North), were the last vestiges of the old Roman Empire. The Vatican, as it's Capital – what headed by not only the leader of The Faith (The Pope), but He served also very much as a traditional Ruler of a Nation. As with any Ruler, He also maintained a small (but effective) Army. This was known as The Papal States Army. They existed from the 8th Century AD until 1871.
Composed of Infantry, Cavalry and Artillery components, The Papal Army was primarily made up of paid Mercenaries from Switzerland, Belgium and Denmark. It did however, later – induct local Italian Volunteer Militia into it's ranks. All of these Forces had to be single and Catholic. The iconic Swiss Guard (protecting The Pope since the 16th Century AD), were left in place inside The Vatican ''Capital'', as was the addition of the previously discussed and recently (then) formed Noble and Palatine Guard.
Styled after The French Army of the day, The Papal Army fought in several key internal Civil Wars of Italy. However, the constant in-fighting for Independence – put too much of a strain on not only The Papal Army, but Italy's Royal Army. With Independence (1870s), in order to preserve The Vatican as an autonomous State, it's Army was dissolved. Only The Noble and Palatine Guard would remain from the old Papal State Army (and then, only until the beginning of the 20th Century).