“In ordinary times there would have been a positive response,” said Robert Malley, the director of the International Crisis Group and a former member of the National Security Council under President Barack Obama. “But now there is a fear of being seen as too closely associated with the United States.’’

The Europeans support the 2015 deal that was intended to prevent Iran from obtaining a nuclear weapon, and have been working to salvage it since Mr. Trump abandoned the agreement last year. Tensions have escalated as Tehran presses for relief from new American sanctions.

‘‘Whatever efforts Europeans make, even if parallel efforts, in Iranian minds it will look like two pieces of one move,’’ he said. ‘‘So it’s hard to disentangle themselves from a U.S. policy they see as having provoked this crisis.”

That much was clear late Wednesday, when the German foreign minister, Heiko Maas, a Social Democratic member of the shaky governing coalition, said that his country would not be joining the Americans.

The Social Democrats, junior members of the coalition led by Chancellor Angela Merkel, have been particularly critical of American policy toward Iran. That includes Mr. Trump’s withdrawal from the 2015 nuclear deal as well as his efforts to force new concessions from Iran by trying to cut off its oil exports.

“Germany will not take part in the naval mission proposed and planned by the United States,” Mr. Maas said. “We are in close coordination with our French partners. We consider the ‘maximum pressure’ strategy to be wrong. We do not want a further military escalation; we will continue to focus on diplomacy.”