During the first months of Donald Trump’s presidency, the U.S. began a historic shift away from Pax Americana, the liberal international order that was established in the wake of World War II. Since 1945, Pax Americana has promised peaceful international relations and an open economy, buttressed by U.S. military power. In championing “America First” isolationism and protectionism, President Trump has shifted the political mood toward selective U.S. engagement, where foreign commitments are limited to areas of vital U.S. interest and economic nationalism is the order of the day. Geopolitical allies and challengers alike are paying close attention.