Alma the Younger was an amazing man. He's probably my favorite character in the Book of Mormon.
I knew Alma was the high priest and the chief judge in the land (Mosiah 29:42,44), until he gave up the latter post (Alma 4:18), but I never thought of him as a warrior. Well, guess what? He fought in the battle against the Amlicites (Alma 2:16). I can't figure out if he was the head of the armies, or if he just went up with the head of the armies, since he was the chief governor at the time.
Later in the book of Alma we see that Moroni is the captain of the armies (Alma 43:16,17), both while Nephihah is the chief judge (Alma 4:16,17), and also when Nephihah's son Pahoran becomes chief judge (Alma 50:39), so the two positions of chief governor and captain of the armies are not necessarily coincidental.
Also, Pahoran is stated to be the chief judge and governor (Alma 50:39), so I think those two positions are, in fact, one and the same.