I was reading a news article about how an ex-C.I.A. official commented on the monitoring of terrorists that travel around the world. His contention is sleeper cells are in the United States.
Back in WW 2, known enemy operatives were either arrested, or killed. Political boundaries were ignored and the enemy was destroyed, which is how you handle enemies that threaten to kill you.
Personally, I wouldn't be disturbed if there were a few well trained individuals taking care of these terrorists as they roamed about. Them, those that help finance their activities, and those that aid them in any way. In my opinion, they're like rabid animals. The rules of law don't apply to such and the world is a better place when they're eliminated.