He took an oath when he joined up the first time. Part of that oath was the possibility that the United States could mobilize him later, even after he left the service. Nobody ever expected that said provision would ever be used in anything less than actual invasion of the United States, but that's not what the oath said.

And he looks at his word the way you look at yours.

Okay, so I can't imagine you taking an open-ended oath of service to a country, no matter who controlled that country, although I can imagine you taking an oath to a person -- but that's what Kent did.