“This would give Europeans even less breathing room to keep the J.C.P.O.A. alive until November 2020,” when the world would know whether President Trump is re-elected, Ms. Geranmayeh said. “It will be very difficult for the Europeans to hold it together.”

After conversations on Friday morning with officials in Tehran, she said that “people inside Iran pushing for the diplomatic process the Europeans have been advocating have lost a lot of ground.”

She said she expected the Iranian response to the killing to be on different tracks, potentially including military attacks on American targets in Iraq, Syria, Afghanistan or elsewhere; cyberattacks; or various other possibilities.

Part of the Iranian debate, she said, is whether to retaliate openly or through proxies in a more deniable way, which has been Iran’s practice in the past. “But with the internal pressure building up inside Iran, and now this external pressure, Iranian decision makers will have to let off some steam somewhere,” Ms. Geranmayeh said.

Carl Bildt, the former prime minister of Sweden, said that Europe had been trying to prevent war between Iran and the United States for a decade, but that the scope for salvaging the nuclear accord now “is very small, and the scope for diplomacy is extremely limited.”