For a student of military history, the most astonishing fact about the current international scene is that there isn't a single conflict in which two uniformed militaries are pitted against each other. The last one was a brief clash in 2008 between Russia and Georgia. In our day, the specter of conventional conflict, which has dominated the imagination of the West since the days of the Greek hoplites, has almost been lifted.

But the world is hardly at peace. Algeria fights hostage-takers at a gas plant. France fights Islamist...