My tiny little mind tends to work through metaphors. In this case, I find a biological one useful. There are pathogens floating around all the time. But they are the most successful and do the most damage when the host is weakened by something else.

The American approach to counterinsurgency tends to be bringing the existing infection under control but, because it is very hard and expensive, we seldom eradicate the factors that weakened the host system in the first place. This means that a recurrence of the infection--a "resurgency" (which is something I intend to write about in the future)--is possible or even probable.