In the eyes of Mike Pompeo, the day was shaping up to be one of his most commanding displays of diplomacy since becoming secretary of state. Months of planning had finally yielded a meeting among reluctant European officials, Arab leaders and the Israeli prime minister to strategize over confronting Iran.

President Trump’s threat to rip up the Iran nuclear deal has touched off an urgent scramble in European capitals to preserve the agreement — not by rewriting it, but by creating a successor deal intended to halt Iran’s ballistic missile program and make permanent the restrictions on its ability to produce nuclear fuel.