Nuclear weapons are the worst military threat that America faces. A single 500-kiloton warhead exploded over a major U.S. city would destroy or heavily damage all buildings to a radius of three miles, and cause widespread fires to a radius of six miles. Electric grids and other fragile infrastructure would fail for tens of miles in every direction. If it was a big city like New York or L.A., prompt and delayed fatalities would likely exceed a million people. Radiation would render much of the damaged area uninhabitable for years.