Primarily, they are concerned about the remote but real possibility that tensions could escalate to the point that Trump pulls a card that Iran can’t: using a new line of “tactical” nuclear weapons to strike a large or subterranean target. Though there have been decades’ worth of test explosions, nuclear weapons were first and last used in combat against Japan on the cities of Hiroshima and Nagasaki in 1945.

“In any other circumstance, I would have argued that the norm against using nuclear weapons is so strong there’s no chance that a president would use a nuclear weapon,” Jeffrey Lewis, a professor at the Middlebury Institute of International Studies at Monterey who studies nuclear arms control, told Business Insider. “At the end of the day, though, it’s just a norm. And this president delights in smashing norms.”