Nearly 3,000 people were killed five years ago when terrorists plowed airplanes into New York’s World Trade Center, the Pentagon, and a field in Pennsylvania. The terrorist attack was undoubtedly a terrible tragedy. But it could have been much worse. Eight years earlier, aides to Osama bin Laden met with Salah Abdel al-Mobruk, a Sudanese military officer and former government minister who offered to sell weapons-grade uranium to the terrorists for $1.5 million. He proffered up a 3-foot-long cylinder. The al Qaeda representatives agreed to the purchase, because after all, as one of them later said, “It’s easy to kill more people with uranium.”

The cylinder turned out to be a dud. But had it actually contained highly enriched uranium, and if bin Laden’s deputies had managed to use it to assemble, then transport and detonate a nuclear bomb, history would have looked very different. September 11 would be remembered as the day when hundreds of thousands of people were killed.

Osama bin Laden’s long-standing interest in developing nuclear weapons is deeply troubling, and the attempt to purchase uranium from the Sudanese was far from an isolated incident. Al Qaeda operatives have repeatedly tried to acquire nuclear materials over the years. In August 2001, a month before the September 11 attacks, bin Laden received two former Pakistani nuclear officials, asking them to help recruit other Pakistani scientists with expertise in building nuclear weapons. After the military effort to oust the Taliban from Afghanistan, U.S. forces found extensive documents, including crude bomb designs, at an al Qaeda safe house in Kabul. In 2003, bin Laden sought a fatwa from an extremist Saudi cleric permitting the use of weapons of mass destruction, calling their acquisition a “religious...