On Oct. 29, one week before the U.S. imposed sweeping economic sanctions against Iran, flag carrier Iran Air dispatched 11 international flights. Two weeks later, a U.S. ban on the airline firmly in place, it flew 13—touching down in destinations including Paris, London, Hamburg and Doha.

The failure of sanctions to slow down Iran Air points to the challenge facing the Trump administration in its campaign to use international isolation to pressure Iran.

The...