David Cameron today told skeptics in Parliament that Britain should stay in the European Union because, "we have the best of both worlds." But the Prime Minister's credibility is being challenged as British voters prepare for a highly unusual referendum, and the outcome is increasingly uncertain. The US needs Britain to stay. So do France and Germany. But the refugee crisis and terrorism on the mainland are reminders that political unity can be costly. We hear what's at stake for international diplomacy and the global economy.