It may be true that no good deed goes unpunished, but only the ever-active Donald Trump could take it upon himself to punish his own good deed.

The good deed was ordering the elimination of Iran’s Qasem Soleimani, who as head of Iran’s Quds Force spent his years deploying one strategy: Export the 1979 revolution by killing people. The dead included Americans, Iraqis, Iranians, Europeans, Syrians and others across the Middle East. In the days before a drone hit his car, Soleimani was planning more death.

President Trump had a good couple of days after the strike. His statement on the action was measured and direct. Despite criticism, Mr. Trump kept quiet, letting the action speak for itself. Still, it was a major decision by the President involving the nation’s interests, and naturally his supporters across the country wondered what would come next. What came next was something familiar: a Trumpian crackback at his critics–in Iraq.

Though Mr. Trump kept the lid on Sunday as pundits and Democrats howled across social media and the morning shows, he apparently couldn’t abide a largely symbolic vote in Iraq’s parliament–most Sunnis and Kurds didn’t show up–to expel U.S. troops from the country. Aboard Air Force One Sunday evening, Mr. Trump threatened to “put very big sanctions on Iraq.” His twin threat to bomb Iran’s “cultural sites” quickly devolved into a debate about the Geneva Conventions, with some Republicans separating themselves from the remarks.

We think the President’s strike against Soleimani was justified on the merits, as Secretary of State Mike Pompeo spent Sunday morning explaining on TV. A concurrent reality, however, is that we are starting a presidential election. To win, the Democrats desperately need to be able to run against Mr. Trump personally, as Mike Bloomberg’s ad blitz is making clear.

If the President allows his Soleimani decision to look like a one-and-done event, with no follow-up beyond tweets and rhetorical barrages against the Iranian and Iraqi people, he’ll give his opponents an opening.

Bernie Sanders and Elizabeth Warren may be on the dismissable fringe of Democratic foreign policy, but moderates such as Joe Biden, Amy Klobuchar and Mr. Bloomberg will seek to buttress their “return to normalcy” argument by saying Mr. Trump’s post-Soleimani behavior shows he is too impetuous and volatile to entrust with national security. They know their best chance lies with driving voter unease about Mr. Trump as Commander in Chief.

Mr. Trump’s obligation is to prove them wrong. Isolationists in his party will counsel Mr. Trump to wash his hands of the post-Soleimani world, but that isn’t possible now. With that decision, President Trump has put powerful forces in play in the Middle East and beyond. If events now spin in dangerous ways, such as if the U.S. leaves Iraq in a huff, Mr. Trump will not be able to blame everyone else. He should be reassuring Iraq that the U.S. is there to help preserve its sovereignty, not to exploit it.

The Iranian mullahs’ threats against U.S. citizens may or may not be bluster. Their announced intention to abandon limits on uranium enrichment under the Obama nuclear deal isn’t much more than they were already doing. But it is meant to drive a wedge between the U.S. and Europe while they wait for Mr. Trump’s successor in 2021.

Since pulling out of that pact in 2018, Mr. Trump has developed an increasingly strong hand with a “maximum pressure” campaign built around severe economic sanctions on Iran. The mullahs are unloved at home and have few real outside allies. Their cat’s paw, Qasem Soleimani, is gone.

The opportunity now exists to shape a coalition of allies, and perhaps even a few serious Democrats, in support of additional policy initiatives on Iran. We would not rule out proposing talks with the Iranian regime about negotiating an end game to its self-depleting 40-year struggle with the West.

Targeting Soleimani was a bold act that other Presidents probably would not have attempted to restore a measure of deterrence against an enemy state. Most Americans appreciated its show of strength. But now Mr. Trump has to show he can manage the consequences in a way that proves it was a wise decision in America’s interests.

The Wall Street Journal