At risk is a possible American or Israeli military strike on Iran’s nuclear sites, or the Iranians following through on their threats to pursue an atomic weapon.

That the talks are even happening represents a historic moment, given the decades of enmity between the two countries since the 1979 Islamic Revolution and the U.S. Embassy hostage crisis.

ROME—Iran is planning to set out a series of proposals for a new nuclear pact, including guarantees from the Trump administration that the U.S. won’t leave a future accord, people familiar with the matter said,