Terrorism is the current justification for military operations abroad. Likewise, there are some who believe terrorism to be an externality of our operations abroad, in which case there is positive feedback. We are told by our leaders as the justification for war that it will cause negative feedback, though the recurrence of terrorism would suggest this is not the case. Even without any feedback, to look at this economically you would have to take into account the news, the journalists, the wars, the arms sales, the rebuilding, the cable-laying that comes from war and the attempt at peace.
Consider two countries at war with the other. Now imagine that both are part of the same territory but the planes still fly and the bombs still fall on the same places. It is clear that this is economic and political madness. So as a combined system, the two warring countries are better off not engaging in war. But if one country can make money from war then it is beneficial for it to be at war.
The answer is for those who govern to think in terms of the whole system, i.e. both countries. That is unpopular with patriotic voters who accuse such leaders of being traitors.