Now this is by no means my specialty but I do know of a couple specific instances. What you have to understand is that in Medieval Europe many Ecclesiastical positions, such as Bishop or Archbishop, were just as much a political and temporal appointment as they were religious or spiritual. Leading clergymen held large swathes of land, with thousands of tenants and subtenants. In fact some of the greatest feudal lords in Medieval England were churchmen - the Bishopric of Winchester was perhaps one of the richest seats in the entire country.

Knowing how the feudal system worked militarily, it essentially relied upon the King (or Duke or Emperor or whomever) being able to call upon his feudal lords to serve him in battle. If this were a Duke, the Duke would then ride to war with a certain amount of men (usually dictated, at least in Feudal England, by the amount of arable land under said Duke) and so on down the chain. Because a sizeable portion of land in England (or any other European country, for that matter) was controlled by the various dioceses, the King was not about to allow such a formidable portion of his total military strength not serve. Thus in many cases (especially in the 11th Century and before - Clergymen were less likely to be in combat after the 1st and 2nd Lateran councils of the 12th Century) you have high ranking clergymen going to war, leading contingents drawn from their extensive holdings. Another thing to consider is that many leading Clergymen were also Feudal lords of completely secular lordships. Perhaps the best example is Odo of Bayeux, the brother of William the Conqueror. Odo was ordained as the Bishop of Bayeux but is known to history more as a warrior than as a statesman. In fact he also eventually became the Earl of Kent, thus making him responsible to the King for not only his ecclesiastical lands, but also those that he held primarily as a feudal lord. Odo was present at the Battle of Hastings and many other confrontations, and so he fits well into your description of a high-ranking Clergyman who led troops into battle.

The practice of Bishops also holding significant secular lordships was so common, that there is actually an anecdote that mocks the situation. I'm going to butcher the story, and I forget the specific King, (it was 12th Century England for sure) but essentially the King wished to imprison one of his Bishops for acting against him. His counselors suggested he do so. His retort was "You wish me to imprison the Bishop of Winchester?" One counselor quipped "No, I suggest you imprison the Earl of Somerset." The joke being that the King would be imprisoning a secular lord instead of a clergyman, despite the Earl and the Bishop being the same person.

Anyway the idea is that many leading clergymen were either political leaders who were expected to lead armed retinues into battle (at least in some cases) or also secular lordships in addition to their ecclesiastical ones, and so performed military service for those lands.

It should be added that I'm not aware of many examples of low-ranking priests being in charge of military forces in the middle ages. I'm sure there are some, and perhaps some of the other AskHistorian people can fill you in. Usually if a Clergyman was leading forces, then he was at least a Bishop, which was a very high-ranking figure in medieval Europe.

For Odo of Kent, I would recommend giving this a quick readthrough: Bates, David (September 2004). "Odo, earl of Kent (d. 1097)". Oxford Dictionary of National Biography It's pretty basic but it will give you a good outline.

Also Frank Barlow's Feudal Kingdom of England, 1042 to 1216 is a great book that will give you several instances of powerful Bishops and Archbishops wielding power politically and even militarily as well as spiritually.

I hope this at least partially answered your question!