If America is attacked, the strike probably won’t come from North Korea nor is it likely the nuke would be delivered by an intercontinental ballistics missile.
More than likely, a nuclear attack on an American city would be carried out by a terrorist group using an off-the-shelf nuclear bomb left in a parked vehicle.
In 2002, “without resorting to any illegal activities or drawing on classified information, and using only commercially available parts,” several nuclear laboratories “built a nuclear bomb that was ‘bigger than a breadbox but smaller than a dump truck.’”
According to New York Magazine:
When such a bomb explodes, everyone within 100 feet of ground zero is instantaneously reduced to a spray of atoms. There are photos from Hiroshima and Nagasaki showing eerie silhouettes of people cast against a flat surface, such as a wall or floor. These are not, as is sometimes claimed, the remains of vaporized individuals, but rather a kind of morbid nuclear photograph. The heat of the nuclear explosion bleaches or darkens the background surface, except for the spot blocked by the person, leaving a corresponding outline. In some cases the heat released by the explosion will also burn the patterns of clothing onto people’s skin.
Near the center of the blast, the suffering and devastation most closely conform to the fictional apocalypse of our imaginations. This is what it would look like within a half-mile of Times Square: Few buildings would remain standing. Mountains of rubble would soar as high as 30 feet. As fires raged, smoke and ash would loft into the air. The New York Public Library’s stone guardians would be reduced to pebble and dust. Rockefeller Center would be an unrecognizable snarl of steel and concrete, its titanic statue of Prometheus — eight tons of bronze and plaster clad in gold — completely incinerated.
Within a half-mile radius of the blast, there would be few survivors. Those closest to the atomic bombings on Hiroshima and Nagasaki have described the horrors they witnessed: People with ripped sheets of skin hanging from their bodies; people whose brains were visible through their shattered skulls; people with holes for eyes. Sakue Shimohira watched her mother’s charred body crumble into ash as she tried to wake her. Shigeko Sasamori’s father cut off the blackened husk of skin all over her face, revealing pools of pus beneath.
As the fireball travels outward from the blast, people, buildings, and trees within a one-mile radius would be severely burned or charred. Metal, fabric, plastic, and clay would ignite, melt, or blister. The intense heat would set gas lines, fuel tanks, and power lines on fire, and an electromagnetic pulse created by the explosion would knock out most computers, cell phones, and communication towers within several miles.
Traveling much farther than the fireball, a colossal pressure wave would hurtle forth faster than the speed of sound, generating winds up to 500 miles per hour. The shock wave would demolish the flimsiest buildings and strip the walls and roofs off stronger structures, leaving only their naked and warped scaffolding. It would snap utility poles like toothpicks and rip through trees, fling people through the air, and turn brick, glass, wood, and metal into deadly projectiles. A blast in Times Square, combined with the fireball, would carve a crater 50 feet deep at the center of the explosion. The shock wave would reach a diameter of nearly 3.2 miles, shattering windows as far as Gramercy Park and the American Museum of Natural History.
All this would happen within a few seconds.