And why Iran should worry
US President Donald Trump was briefed while the missile was still in the air
Missile launch comes a day after N. Korea threatened to turn US into "ashes and darkness"
Japanese public broadcaster NHK said the coast guard issued safety warnings to aircraft and ships
"That range would not be enough to reach the lower 48 states or the large islands of Hawaii, but would allow it to reach all of Alaska"
The exercise marked a significant step for the GMD system, which has had a checkered record in previous tests