Could the U.S. take a tougher line?

While the scale and target of Biden’s promised response is not yet clear, any unilateral move is likely to draw blowback from key allies in the Middle East who worry about sparking a regional war.

Saudi Arabia has pushed for restraint in dealings with Tehran and fears the economic cost of regional instability.

Turkey, a key NATO ally, has denounced Israel’s campaign in Gaza, while President Recep Tayyip Erdoğan has accused the U.K. and the U.S. of trying to turn the Red Sea into a “sea of blood.”

“Turkey does not want to be drawn into this conflict because it shares a border with Iran,” said Selin Nasi, a visiting fellow at the European Institute of the London School of Economics. “If the U.S. as its main ally in NATO gets involved in this military conflict directly then Turkey has to choose a side, and that will mean it’s harder to maintain a balanced approach — like it has done with the war in Ukraine.”

The challenge for Biden is how to retaliate without risking escalation by Iran and its partners in the region. Conversely, doing nothing — especially after having said he would avenge the deaths of the three U.S. soldiers — would leave him vulnerable to a charge of weakness from Trump.

“Iran’s leadership probably calculates that the United States will be reticent to fulsomely respond in any manner that would risk escalation of tensions in the Middle East and spark the region-wide [conflict] the Biden administration has admirably tried to prevent the past three months,” said Jonathan Panikoff, a former U.S. deputy national intelligence officer.

But the U.S. may have “to undertake a more fulsome response to restore deterrence,” he added.

Jamie Dettmer, Jeremy Van der Haegen and Laura Kayali contributed reporting.