At the risk of spoiling, I'll spare everyone the speculation. Mulligan doesn't get killed in action. Don't worry about him, he'll be just fine, but we'll never see him again. That's just how it goes.
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I didn't think Mulligan would end up being KIA, based on Peter's last comment in the previous page. It sounded more like he hasn't seen him since, but he knows Mulligan's still out there and they just haven't crossed paths again yet. Maybe Mulligan took to the service better than anyone expected and became a 'lifer'.
I had a friend whom Mulligan reminded me a lot of, and he did exactly what Mulligan did: shocked the hell out of a lot of people by joining the Army. I haven't seen him in probably 20 years, but last I heard he'd become a 'lifer'. Who knows if or when we'll cross paths again, but like Peter, I know he's still out there, so that possibility still exists.