Mr. Trump tested that balance last year when he reimposed oil sanctions on Iran, while exempting some of its largest customers. The result, according to the administration, was to shave $10 billion off the Islamic Republic’s prior $50 billion in oil revenues, which in turn led to funding cuts for the Iranian-backed Hezbollah movement in Lebanon. Despite threats, Tehran did not respond to the losses by crossing U.S. red lines.

Perhaps encouraged by this passivity, the administration is doubling down, seeking to eliminate Iran’s remaining oil exports by threatening buyers, including China, India, Turkey, South Korea and Japan, with sanctions if they do not comply. China, which buys half the remaining oil, may defy U.S. pressure. But if the initiative succeeds, the already-struggling Iranian economy would be devastated. Mr. Trump is essentially betting that the regime will again swallow the punishment rather than test his resolve.

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It’s possible that Ayatollah Ali Khamenei will choose that course; the strong opposition of European governments and U.S. Democrats to Mr. Trump’s policy may encourage the regime to wait out the next 18 months in the hope he will not be reelected. But if Iran responds aggressively, Mr. Trump could be forced to choose between tolerating offenses that the Obama administration’s 2015 nuclear deal stopped or sliding toward a war that few Americans wish for.

One reason confrontation is possible is that the administration has left Tehran with scant options. Secretary of State Mike Pompeo on Monday reiterated the administration’s nominal openness to negotiations, but he has set out a list of a dozen conditions the regime would have to meet, representing a complete reversal of its current foreign policy. Those steps, ranging from a stop to the development of nuclear-capable missiles to an end to hostility toward Israel, are desirable but unrealistic in the absence of a broader settlement with Iran’s regional rivals, including Saudi Arabia.

Maybe the administration’s real objective is to force the regime’s collapse. Mr. Pompeo said his demands “are similar to what we hear from the Iranian people themselves,” adding “we will not appease their oppressors.” That, too, could be a welcome outcome, were U.S. sanctions capable of delivering it. History doesn’t offer much encouragement, however, which raises the question of whether Mr. Trump has carefully considered the difficult corners into which his policy may lead. We’re guessing he has not.