Watching the events cascading in Syria makes it eerily easy to see how the political elites of 1914 stumbled into World War I while believing they were pursuing a sensible set of national interests. The parallels are far from precise. The alliances bonding the players in today’s Middle East aren’t as interlocking as those in early 20th-century Europe. The war-mobilizing machinery isn’t as rigid. And, of course, today’s leaders have the precautionary example of World War I to rivet their attention: They know the pitfalls of escalation and the tragic consequences of unbounded warfare—though people don’t always heed the lessons...