But rather than take responsibility for deciding the future of the agreement, the president wants to pass the buck. The White House has signaled that after his certification decision, Mr. Trump will urge the Republican-controlled Congress not to reimpose sanctions on Iran that would scuttle the deal. Instead, he hopes Congress will pass new legislation to address concerns that were never part of the nuclear agreement’s original mandate. If Congress complies, such unilateral action to change a multilateral agreement will effectively kill it.

The president’s unwillingness to accept the truth about the Iran deal — that it is working to keep Iran from obtaining a nuclear weapon and that it is clearly in America’s national security interest — will have far-reaching consequences.

For one, this decision will breach the trust of America’s partners and isolate our country. The deal was agreed to by the five permanent members of the United Nations Security Council — the United States, Britain, France, Russia and China — along with Germany and the European Union. It was then ratified unanimously by the full Security Council. All of these parties, except the United States, want to keep the accord in place.

If President Trump undermines the nuclear deal, the repercussions for American foreign policy will be disastrous: It will drive a wedge between the United States and Europe, weakening the critical trans-Atlantic relationship and increasing the influence of Iran, Russia and China. And when the president travels to China next month seeking support to deal with North Korea’s nuclear program, he will find the Chinese less willing partners. Washington’s credibility will be damaged for the next time we want countries to agree to something, such as condemning Iran’s malicious behavior in the Middle East or tightening the screws on North Korea. Indeed, we are likely to lose any possibility of dialogue with North Korea because Pyongyang will assume the United States will not honor its commitments, even on multilateral agreements. Unpredictability — a favorite self-justification for the president’s erratic actions — has its place as a negotiating tactic, but when it comes to war and peace, reliability and credibility matter most.