It is no exaggeration to say that the postwar effort to build a liberal democratic Europe has been America’s most successful foreign policy achievement, helping to ensure peace and prosperity on a continent once racked by total war, genocide and economic privation. That consistent, bipartisan commitment to a “Europe whole, free and at peace” is at stake now that Donald Trump is president of the United States. Like no American leader before him, Trump has questioned the very foundations of transatlanticism, openly rooting for the dissolution of the European Union and repeatedly denigrating NATO as “obsolete.”