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

Harvard Kennedy School (HKS) and the German Council on Foreign Relations (DGAP) convened a strategy group of experts and former government officials from the United States and Europe over the past year to discuss the crisis in the transatlantic relationship and to propose a strategy to revive and strengthen it.

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.

President Trump and his aides have sent a dizzying, seemingly conflicting set of messages to Iran in recent weeks, ordering more troops to the Middle East and a carrier to the Arabian Sea as military threats even while declaring that Washington is seeking new negotiations, not war.