resident Barack Obama flies Sunday to a Europe shaken by Vladimir Putin’s seizure of Crimea and his threats against eastern parts of Ukraine. At a summit conference at The Hague on preventing nuclear terrorism – scheduled long before the Ukraine crisis – Obama will confer with German Chancellor Angela Merkel and other leaders. On Monday afternoon Obama will meet with President Xi Jinping of China, Russia’s neighbor and sometime rival. Putin has forced the United States and Europe to refocus on the origins of the modern transatlantic alliance -- the 1949 NATO defense partnership designed to prevent a Russian invasion...