Have to say, David Frum seems dead wrong about this. As I understand it, an incipient civil war is one in which a few civil-war-like activities are taking place (Kurds and Arabs skirmishing in Kirkuk and Mosul, say), but nothing major quite yet, and hardly inevitable. An imminent civil war is one in which full-blown fighting is only weeks or days away. The former is troublesome; the latter is a catastrophe.

The key to these definitions, I think, is that the prototypical "civil war" is a full-blown one, with lots of people dying and countrymen squaring off against each other en masse. That's what we generally mean when we say "civil war". So the adjective "incipient" explicitly modifies only the size and scope of this concept—namely, by reducing it. But the adjective does not imply that the civil war is likely to start anytime soon, because we don't really have a prototypical or commonly shared notion of how rapidly civil wars unfold.

"Imminent", meanwhile, specifies only the civil war's point in time, which leaves us to imagine a prototypical civil-war -- i.e., a big one -- about to start any day now. I don't think that's a perfect explanation -- cognitive linguistics isn't my strong suit -- but it seems to get at the difference.