"I'm sure most, if not all of you know that most Arab armies are crap. But, when Arabs fight, in accordance with their own traditions, which is irregular light cavalry warfare, they've proven quite good at it. We call it terrorism. The passing way of war always calls the arriving way of war terrorism. To the knights on horseback, musketeers were terrorists. There were actually rules ordering their eyes to be put out if they were captured."
-William S. Lind, The Four Generations of Modern War
Designating an enemy as "terrorists", no matter how brutal and wicked they are, is really a protest among the other players that such-and-such is not playing by the rules. I think it's also a signal of weakness or inability to cope with a defection from the rules. But you can't take your ball and go home in geopolitics. Peace is a goal, but war is a fact. And the rules of the dar-al-harb are written by the aggressor. The rules of Pax Americana only stood while we were able to enforce them. Those days ended in 'Nam.
We endanger ourselves when we can't handle being engaged other than by our own rules. This is why we have borders, actually, or one reason. Familial, social, and national. Borders tell us what rules apply, and how we can expect to be engaged.