Iran’s most heavily fortified nuclear site is at Fordo, a hilly area south of Tehran and about 15 miles away from Qum, a city where more than a million people live.

The move, the first time the U.S. has attacked Iran, is a major escalation of the conflict between Israel and Iran.

It was an unprecedented attack years in the making, with some last-minute misdirection meant to give the operation a powerful element of surprise.