I believe there is a basic flaw in the question, namely "One political group or the leadership of one nation" can be two different things (possibly three) and call for at least two different responses (possibly three).

1. If the leadership of the attacking nation has the support of the nation's people, then that would appear to be classic war, and the response is fairly clear-cut. One defends oneself.

2. If the leadership of the attacking nation does not have the support of the nation's people, or at least, not a large percentage of that nation's people, then the problem is a bit more complex. One defends oneself, and one viable tactic in that defense would be to determine if there is an underground in the attacking nation, and if so, attempt to form an alliance, enlist their aid, and do as much as you can to assist them.

3. If it is a "political group," and not a discreet nation, then you have a situation where classic war is rarely, if ever, the answer. This sort of thing is more like tracking down criminals than it is war, and requires entirely different tactics. If a given nation actually harbors and gives aid to the group, then presumably the attack was with the approval of that nation. If the leaders of that nation are unwilling to cooperate in efforts to bring the criminals to justice, then (as Rabbi-Sol has said) the "doctrine of hot pursuit" is perfectly in order. This would not be all-out invasion, it would be more in the nature of surgical commando strikes. At the same time, one should seek the assistance of other nations in attempting to restore order and bring the criminals to book, including the use of international sanctions against any nation or nations that harbor or give aid to the criminals. One certainly does not attack and/or invade nations that (even though they may be in sympathy) do not harbor or actively give aid to the criminals.

This sort of action is not "war" and should not be characterized as such. It gives a totally false impression, it can easily (as we currently see) lead to actions which go into left field, cause all kinds of international complications, put one's own nation in a highly questionable position in terms of international law and moral rectitude, and do nothing to solve the original problem—in fact, it can make it much worse. And to call it "war" empowers and lends dignity to the criminals—or terrorists—or whatever you care to call them.

I don't think they teach the correct countermeasures for this kind of criminal activity in most military schools. They're still fighting wars from yesteryear. And political leaders who are thinking only of the next election and seeking only simple answers to complex problems, are totally incompetent to handle problems like this.

Don Firth