If Iran or Iran-backed militias in Iraq do attack American interests at too late a stage for Trump to react, or early in the Biden presidency, there will be a tension between quickly achieving deterrence and establishing U.S. credibility against a careful weighing of the facts and options.

If the attack is powerful enough to kill Americans—who are typically well-protected—then it may have received a go-ahead from Iran, but the incoming administration can wait to ascertain that connection. Clinton waited 72 days until he struck Iraq’s intelligence service, for their role in the attempt on Bush’s life in 1993.

If evidence does emerge of an Iranian role, then a Biden administration—like Clinton’s before, but hopefully quicker—must not flinch from doing the right thing to deter future attacks on Americans. Only if Iran believes this to be the case will they hesitate against striking U.S. personnel in the first place.

What the U.S. could and should do straight away is to signal a cost to any attack on Americans, hit back at Iran’s extended network and retain the option of further, more expansive strikes.