A presidential campaign is an audition for the most challenging job in the world. But the Democrats vying for their party’s nomination don’t seem to understand what that job really is. People with White House experience know that presidents spend more than half their time—often much more—dealing with foreign policy and national security. Thus far, Democratic candidates have had little to say about these issues.

Even in ordinary times, this would be a mistake—and these are not ordinary times. President Trump has shaken the architecture of the world order the U.S. took the lead in creating after World War II.

Mr. Trump regards U.S. alliances as entangling rather than empowering and allies as free-riders rather than friends. He sees multilateral institutions as devices for reducing America’s power and restricting freedom of action. He views foreign and defense policy, even joint military exercises, through the prism of profit-and-loss statements. He does not care whether other countries are democracies or autocracies, so long as their leaders say nice things about him. Indeed, he prefers dealing with leaders who can make binding decisions on their own rather than those who must take into account the views of their people and elected representatives.

The U.S. faces a choice, Mr. Trump insists, between nationalism and globalism—between those who put America first and those who subordinate Americans interests to those of other peoples. This is a false choice, of course. Other than a handful of philosophers and international bureaucrats, no one thinks that national leaders should give equal favor to nonmembers of their communities. When nations have competing interests, one’s own citizens come first.

The real choice is not between national interest and universalism; it is about different conceptions of the national interest. Seven decades of American leaders in both political parties believed that America was strongest and securest when maintaining democratic alliances and international institutions, which define the rules of engagement among nations.