At 11:15 on a Monday morning in May, an ordinary looking delivery van rolls into the intersection of 16th and K streets NW in downtown Washington, D.C., just a few blocks north of the White House. Inside, suicide bombers trip a switch.

Instantly, most of a city block vanishes in a nuclear fireball two-thirds the size of the one that engulfed Hiroshima, Japan. Powered by 5 kilograms of highly enriched uranium that terrorists had hijacked weeks earlier, the blast smashes buildings for at least a kilometer in every direction and leaves hundreds of thousands of people dead or dying in the ruins. An electromagnetic pulse fries cellphones within 5 kilometers, and the power grid across much of the city goes dark. Winds shear the bomb's mushroom cloud into a plume of radioactive fallout that drifts eastward into the Maryland suburbs. Roads quickly become jammed with people on the move—some trying to flee the area, but many more looking for missing family members or seeking medical help.

It's all make-believe, of course—but with deadly serious purpose. Known as National Planning Scenario 1 (NPS1), that nuclear attack story line originated in the 1950s as a kind of war game, a safe way for national security officials and emergency managers to test their response plans before having to face the real thing.

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