Iran and the United States started a new round of nuclear talks in Rome on Saturday to resolve their decades-long standoff over Tehran's atomic aims, under the shadow of President Donald Trump's threat to unleash military action if diplomacy fails.

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.

Iran and the United States are preparing for a second round of negotiations over Tehran’s rapidly advancing nuclear program in Rome.