This can't be stressed enough. War is not a legal exercise. Your enemy is not your enemy for some legal infraction, in fact what he does is likely perfectly legal by his code of conduct, and if his legal advisors are sufficiently sharp, it may even emulate legality under your code.
He is not at war with you because of some legal infraction, he is at war with you because he wants to destroy or subjugate you, and he is prepared to use any means to do it.
War is the state of affairs that exists when law is insufficient to resolve a conflict, war is what exists when your opponent is outside the jurisdiction of your laws, or does not recognize your laws, or is prepared to use your laws to destroy and enslave you.
In the moment you recognize your mortal danger, you must act, you must be prepared to do whatever it takes, and "whatever it takes" means just that, whatever it takes. You aren't going to limit yourself to serving subpoenas on your enemies, you are going to round them up, you are going to deport them, imprison them for as long as seems prudent, you are going to target them and kill them and anyone standing near them, you are going to identify and destroy their supporters, and you are going to do it with as much grace or brute force as you deem necessary.
These guys should not be on trial. They should be in a holding camp, being sweated for information so you can find their pals and get them too.
And, once you've goten all the information they have, have them dig a deep hole.
ping