In 1983, as the U.S. confronted the threat posed by the Soviet Union, President Ronald Reagan explained America’s unique responsibility. “It is up to us in our time,” he said, “to choose, and choose wisely, between the hard but necessary task of preserving peace and freedom, and the temptation to ignore our duty and blindly hope for the best while the enemies of freedom grow stronger day by day.” It was up to us then—as it is now—because we are the exceptional nation. America has guaranteed freedom, security and peace for a larger share of humanity than any other nation in all of history. There is no other like us. There...