Mr. Obama cites opinion polls purportedly showing that Americans are “war weary.” Probably what the polls really reflect is something else entirely, dismay at the wasted blood and treasure that resulted from Mr. Obama’s unilateral declaration of defeat in Iraq and Afghanistan.

Instead of whining about “war weariness,” an American president should understand his historical role. The U.S. can’t just withdraw from the responsibilities that have derived from its enormous success in making itself the look-to nation for peoples aspiring to safer, freer and more prosperous lives. The costs of failure are too high, as we have seen in the many thousands of lives lost in Syria.

U.S. policy will continue to be measured not only by its willingness to fight but by how effectively it moves to counter troublemakers before trouble happens. An effective president would call a halt to U.S. disarmament, rather than citing it as an accomplishment. He would move to strengthen the hands of America’s friends, like the new Ukrainian government and the Kurds of the Middle East, by providing them with economic and military aid. He would abandon the disastrous policy of trying to schmooze and appease cutthroats like Vladimir Putin.