By Andrew Moravcsik - The outcome of the United Kingdom referendum on EU membership sent shockwaves across the globe. It even caught British voters by surprise. The European Union has helped secure peace in Europe for the past 70 years. Now it faces an uncertain future. Amid a refugee crisis, lingering financial recession and the constant specter of terrorism, unity seems more imperative than ever. But the Brexit vote underscores the complexities of integrating an extremely diverse continent. What will post-Brexit Europe look like, and how can U.S. foreign policy adapt?