Yet any nation that has built nuclear weapons and long-range missiles, as North Korea has done, can easily overcome the relatively much simpler technological challenge of warhead miniaturization and reentry vehicle design.
Indeed, North Korean dictator Kim Jong-Un has been photographed posing with what appears to be a genuine miniaturized nuclear warhead for ballistic missiles. And North Korea does, in fact, have two classes of ICBMs—the road mobile KN-08 and KN-14—which both appear to be equipped with sophisticated reentry vehicles.
Even if it were true that North Korea does not yet have nuclear missiles, their “Dear Leader” could deliver an atomic bomb hidden on a freighter sailing under a false flag into a U.S. port, or hire their terrorist allies to fly a nuclear 9/11 suicide mission across the unprotected border with Mexico. In this scenario, populous port cities like New York, New Orleans, Los Angeles, and San Francisco, or big cities nearest the Mexican border, like San Diego, Phoenix, Austin, and Santa Fe, would be most at risk.
A Hiroshima-type A-Bomb having a yield of 10-kilotons detonated in a major city would cause about 200,000 casualties from blast, thermal, and radiation effects. North Korea has tested a nuclear weapon having an estimated yield of 20-30 kilotons. The Defense Department assesses that on January 6, 2016, North Korea may have tested components of an H-Bomb. H-Bombs are much more powerful than A-Bombs and can produce much greater casualties—millions of casualties in a big city like New York.