AS IF a global financial-market meltdown, the deepest U.S. recession in seventy years, an existential crisis in the euro zone and upheaval in the Middle East hadn’t already created enough trouble for one decade, now the unrest and anxiety have extended to some of the world’s most attractive emerging markets. Just in the past few months, we’ve seen a rough ride for India’s currency, furious nationwide protests in Turkey and Brazil, antigovernment demonstrations in Russia, strikes and violence in South Africa, and an ominous economic slowdown in all these countries. Adding to the uncertainty, as the carnage and confusion in Syria remind us, is the fact that there is no longer a single country or durable alliance of countries both willing and able to exercise consistent global leadership. The Obama administration and congressional Republicans don’t want to alienate a war-weary U.S. public by spending blood in the Middle East or treasure in Europe. Europe’s leaders have their hands ...