Actually, joining an enemy military strips you of your citizenship. But even then, it’s not a stable area. You can’t just ring up the Yemeni police to go arrest them and the NYPD is going to laugh at you. So you call on the guys who can reach the area. Problem, they’re the military and this is a war. So even without the first sentence we’re back to using a missile instead of the police because nobody is going to commit suicide for this and we’re not going to let them operate freely on the notion that we’re not allowed to fight that particular enemy.
Which is why joining an enemy military is an automatic loss of citizenship.