Christmas is over, yet the ghosts of dovish presidential candidates past haunt the Democratic primaries. Eugene McCarthy, George McGovern, Michael Dukakis, John Kerry—these are not kindly, benign spirits; they bring great risks for Democrats in November.

The primary contest to date has served mainly to highlight how far left Democrats have lurched on domestic policy in the few years since Barack Obama. The Iran conflict has now exposed a similarly dramatic shift in foreign policy. Where progressives these days lead, even the “moderate” Joe Biden follows.

The targeted killing of Maj. Gen. Qasem Soleimani led the progressive moment to go full flower child. Thanks to desultory debate moderators, Americans until now had little idea how Democratic aspirants would specifically respond to terrorism or other provocations. The Soleimani moment proved clarifying.

Voters now know that a President Bernie Sanders would not take action against Iran or other rogue regimes, no matter how many red lines they cross. Mr. Sanders will take no step that might bring us anywhere closer to “another disastrous war” or cost “more dollars and more deaths.” A President Elizabeth Warren would similarly offer a pass to leaders of U.S.-designated terrorist groups, at least if they have an official title. The Trump strike, she said, amounted to the “assassination” of “a government official, a high-ranking military official.”

The House Progressive Caucus in a Thursday press conference laid out additional aspects of the left’s foreign-policy worldview. Member after member took to the podium to demand legislation that would hem all presidents in from further acts of deterrence. Rep. Ilhan Omar explained that progressives don’t oppose only military force; they also oppose “crippling sanctions,” which “starve the innocent people of Iran.”