Complex sentence: Prior to serving with JAG, he served in the Marines in Infantry before he decided to leave the military for civilian life, he used to be a sheriff deputy in his home state of New Mexico until he was injured and fell into alcohol.
Split into simpler sentences: Prior to serving with JAG, he served as an infantry Marine before he decided to leave the military for civilian life. He was then sheriff's deputy in his home state of New Mexico until he was injured and fell into alcohol.