The growing tension with Iran is merely the latest, most acute example of Mr. Trump’s impulse toward global destabilization. Whether it’s his abandonment of the Kurds in Syria, his antagonism of America’s allies, his coddling of hostile autocrats, his disdain for multilateral agreements or his manipulation of America’s Ukraine policy for his own political gain — a move that led to his becoming the third president ever impeached — this president has given Americans reason to abandon their complacency on foreign affairs and increase their concern about Mr. Trump’s frightening style of leadership.

In just a few weeks, the voting in the Democratic contest for president will begin. Voters must now decide whom they trust not only to work with Congress on cutting health care costs and cleaning up the political system but also to navigate a world that Mr. Trump has helped make increasingly unsettled and unsettling.

Mr. Biden, a former chairman of the Senate Foreign Relations Committee, is making the case that this calls for a candidate with extensive foreign policy experience.

Mr. Sanders, in turn, is touting his antimilitarist credentials as part of his populist platform. “I know that it is rarely the children of the billionaire class who face the agony of reckless foreign policy — it is the children of working families,” he told supporters Friday at an event in Iowa. Mr. Sanders’s campaign is also reminding voters that, unlike Mr. Biden, he did not support the Iraq war in 2003.

Mr. Buttigieg is playing up his military background. “As a military intelligence officer on the ground in Afghanistan,” he said Friday at a campaign event in New Hampshire, “I was trained to ask these questions before a decision is made.”

Experience matters. But perhaps more important are temperament and judgment and the candidates’ philosophies on the use of American power, both hard and soft. Also, the people a president turns to for advice can be as important as his or her own expertise — yet another lesson that past presidents have provided by their failures.

To aid voters, the moderators for next Tuesday’s Democratic debate should set aside time to drill down on everything from what type of advisers candidates would seek out to how they would adjust our relationship with Saudi Arabia to how they would have handled the situation in Syria differently from Mr. Trump — or President Barack Obama.