An argument that a strong, interventionist America remains the world's best hope against the return of international chaos. Kagan, who served in the State Department during the 1980s, sees the U.S. in retreat from its responsibility at a time when its leadership is needed most. "If Obama's policies put a dent in the liberal world order, Trump's statements and actions have been driving a stake through it," he writes. The 20th century elevated America...