Military thinkers discuss the unthinkableEternal vigilance being the price of liberty, Americans—who spent decades war-gaming a Soviet invasion and have taken more recently to daydreaming about “ticking bomb” scenarios—should cast at least an occasional thought toward the only truly existential threat that American democracy might face today. We now live in a unipolar world, after all, in which conquest of the United States by an outside power is nearly inconceivable. Even the best-equipped terrorists, for their part, could dispatch at most a city or two; and armed revolution is a futile prospect, so fearsomely is our homeland secured by police...