The exchanges were all the more notable because until Tuesday night, foreign policy had been a side show in the 2020 campaign. But the targeted killing of the Iranian commander Maj. Gen. Qassim Suleimani this month ended the moment when the six Democratic candidates could avoid addressing some of the most complex and intractable conflicts that they would inherit if elected. Most seemed to be hoping the subject would soon return to more familiar ground, like universal health insurance or universal background checks for gun sales.

It should have been the moment for Mr. Biden to shine: A former chairman of the Senate Foreign Relations Committee, he was among the most experienced in the Obama Situation Room during debates over increasing troop levels in Afghanistan and secretly carrying out cyberattacks on Iran’s nuclear facilities.

He had a moment to look decisive, especially in contrast to Senator Bernie Sanders of Vermont, who was the most vociferous about extracting the United States from what he called “endless wars.” Again, it echoed a phrase that Mr. Trump has often used — or at least did until his decision to kill General Suleimani all but assured a greater American troop presence in the Middle East for years to come.

Mr. Biden was clearly the one with the best grasp of the global situation, but he seemed unsteady at moments, even when describing events in which he was deeply involved. He argued that he was the one who “led the effort” against “surging tens of thousands of troops into Afghanistan,” without noting that Mr. Obama ignored his advice and ordered a surge anyway. On Iraq, he claimed he was the one who engineered the withdrawal of 156,000 troops from Iraq without noting that it was Mr. Bush, just before leaving office, who ordered a troop withdrawal.

But it was on North Korea — a nation that, unlike Iran, is already believed to have nuclear weapons, with American intelligence agencies putting the number at 30 to 60 — that the candidates seemed softer. Curiously, none vowed that the North would have to give up its weapons, a statement that just a decade ago was a staple of any foreign policy discussion by leading presidential candidates, even if it seemed like a pipe dream.

Instead, the candidates seemed to silently acknowledge that disarming the North was all but a lost cause. Instead, they focused on negotiating tactics, without ever quite saying what the goal would be. Mr. Biden vowed he wouldn’t meet Kim Jong-un, the North Korean leader, “without any preconditions,” but never described what those might be.

What was most striking about the discussion was that the candidates talked about the use of diplomacy and military coercion as if they were alternatives, rather than mutually reinforcing tactics to accomplish a strategic end.