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.

As the two sides prepare to meet for trans-Atlantic talks next month in Berlin, the Europeans are demanding a guarantee that Mr. Trump will abide by the add-on deal after it is negotiated and not jettison it on some other pretext. Mr. Trump has called the 2015 agreement that was negotiated by President Barack Obama “the worst deal” ever and has told Britain, France and Germany to fix it by May 12 or he will pull the United States out.

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