The countries will negotiate next week through intermediaries in Vienna to try to bring both back into compliance with the 2015 nuclear agreement.

The killing of Iran’s top nuclear scientist is likely to impede the country’s military ambitions. Its real purpose may have been to prevent the president-elect from resuming diplomacy with Tehran.

President Trump opened a small window for diplomacy with Iran on Wednesday, but combined his words with bald threats that made it hard to see how the two countries could break out of their cycle of confrontation and revenge.

President Trump is pushing Boris Johnson, the incoming prime minister of Britain, toward a hard break with the European Union. But Mr. Johnson could be constrained.