Did we have this in WW2?
i guess i should have read the article more... i think we did except they were prosecuting the enemy
We always have. In the JAG he could be doing any of dozens of duties, but given that he's an AG he's probably a prosecutor. They are a very necessary component of good order in the Army.
Yes. Tangentially, although a fictional account, one of the best movies about a military trial during WWII is "The Caine Mutiny" (1954) with Humphrey Bogart.