Officials stress that simply having the ability to launch an attack does not mean it would be a success. They also say North Korea is not known to have actually deployed any nuclear-tipped missiles.
Tokyo and Washington have invested billions of dollars in what is probably the world's most sophisticated ballistic missile defense shield since North Korea sent a long-range Taepodong missile over Japan's main island in 1998. Japan now has its own land- and sea-based interceptors and began launching spy satellites after the "Taepodong shock" to keep its own tabs on military activities inside North Korea.
For the time being, most experts believe, North Korea cannot attack the United States with a nuclear warhead because it can't yet fashion one light enough to mount atop a long-range ICBM. But Japanese analysts are not alone in believing North Korea has cleared the "miniaturization" problem for its medium-range weapons.
In April 2005, Lowell Jacoby, director of the Defense Intelligence Agency, told the Senate Armed Services Committee that North Korea had the capability to arm a missile with a nuclear device. In 2011, the same intelligence agency said North Korea "may now have" plutonium-based nuclear warheads that it can deliver by ballistic missiles, aircraft or "unconventional means."
The Pentagon has since backtracked, saying it isn't clear how small a nuclear warhead the North can produce.
But David Albright, a physicist at the Institute for Science and International Security think tank, said in an email he believes the North can arm Rodong missiles with nuclear warheads weighing as much as several hundred kilograms (pounds) and packing a yield in the low kilotons.
That is far smaller than the bombs dropped on Hiroshima or Nagasaki but big enough to cause significant casualties in an urban area.
Japan also is a better target than traditional enemy South Korea because striking so close to home with a nuclear weapon would blanket a good part of its own population with the fallout.
Regardless of whom North Korea strikes — with a nuclear or conventional weapon — it can be assured of one thing: a devastating counterattack by the United States.