Yesterday, President Donald Trump announced that he was withdrawing from the Iran nuclear deal, ultimately dismantling the foreign policy achievement that his predecessor Barack Obama established. Not only that, but the decision also isolates the country from its Western allies and sows uncertainty before a risky nuclear negotiation with North Korea. Mr. Trump’s decision, while long anticipated and widely telegraphed, leaves the 2015 agreement reached by seven countries after nearly three years of grueling negotiations in pieces.

Because of this, the United States will now reimpose the stringent sanctions it imposed on Iran before the deal and is even considering new penalties. Iran has stated that it will remain in the deal, which tightly restricted its nuclear ambitions for a decade or more in return for ending the sanctions that had once crippled its economy. So did Germany, France, and Britain, raising the prospect of a trans-Atlantic clash as European companies are facing the return of American sanctions for doing business with Iran. Meanwhile, China and Russia, also signatories to the deal, are likely to be joining Iran in accusing the United States of violating the accord.

President Trump’s move may end up emboldening Iranian hard-line forces, further raising the threat of retaliation against Israel or the United States, and fueling a Middle Eastern arms race and fanning sectarian conflicts from Syria to Yemen. The president, however, framed his decision as the fulfillment of a bedrock campaign promise and as the act of a deal-maker dissolving a fatally flawed agreement. He predicted his tough line with Iran would strengthen his hand as he prepared to meet North Korean leader Kim Jong-un to begin negotiating the surrender of his nuclear arsenal.

Mr. Trump said during an 11-minute address from the White House’s Diplomatic Reception Room, “This was a horrible one-sided deal that should have never, ever been made. It didn’t bring calm, it didn’t bring peace, and it never will.” This announcement was met with a chorus of opposition from European leaders, several of whom lobbied him feverishly not to pull out of the deal for fixes to it that would satisfy him.