In the years after Barack Obama’s presidency, it became an article of faith that one of his central errors in foreign policy was the Syria “red line.” He had said he would attack Syria if it used chemical weapons—but when evidence emerged that it had used those weapons, he pushed the question of intervention to Congress, which declined to act.

“A disaster,” Donald Trump called it at the time. A cause of “generational and reputational damage,” said then-Sen. Marco Rubio (R-Florida). Part of “an incoherent maze” of foreign policy, Pete Hegseth argued a few years later. In ignoring a red line that he had drawn, Sen. Lindsey Graham (R-South Carolina) explained, Obama had risked squandering American credibility around the world.

In the years after Barack Obama’s presidency, it became an article of faith that one of his central errors in foreign policy was the Syria “red line.” He had said he would attack Syria if it used chemical weapons—but when evidence emerged that it had used those weapons, he pushed the question of intervention to Congress, which declined to act.

“A disaster,” Donald Trump called it at the time. A cause of “generational and reputational damage,” said then-Sen. Marco Rubio (R-Florida). Part of “an incoherent maze” of foreign policy, Pete Hegseth argued a few years later. In ignoring a red line that he had drawn, Sen. Lindsey Graham (R-South Carolina) explained, Obama had risked squandering American credibility around the world.

Obama’s red line flip flop looks like the model of careful policymaking compared to what we have witnessed since the Iran war began. Last week, President Trump posted on social media that “If Iran........

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