North Korea has conducted several missile tests, and at least five nuclear tests since 2006 - the last two last year. It is believed to be readying a sixth.
Its missiles can reach South Korea, Japan and Guam. Some 54,000 US troops are based in Japan, and around 28,000 and 6,000 in South Korea and Guam, respectively.
The view from Washington is that given North Korean leader Kim Jong Un's strident anti-American rhetoric, this makes the country's growing missile and weapons capability a significant threat to the US.
Theoretically, Pyongyang's missiles could reach the mainland United States, but experts believe it still lacks the capability of mounting nuclear warheads on them, and guiding them to land with accuracy. Yet, it is inevitable - very possibly under President Donald Trump's watch - that North Korea will be able to target the US mainland with a nuclear missile.
Multiple US administrations have tried a mix of dialogue, sanctions and blandishments with the goal of "denuclearising" the Korean peninsula. But North Korea, surrounded by big powers and convinced of US aggression, has gone ahead regardless.
Most experts believe the time for military action to stop its weapons programme has passed, and any such move now would trigger a destructive war.
While denuclearisation has become somewhat of a pipe dream, the US is still trying to prevent a scenario in which it is vulnerable to a North Korean strike that it could hold at bay only with superior anti-missile technology and its own nuclear deterrent.
The threat of an intercontinental ballistic missile capable of putting a nuclear warhead into the US is closer now than at any time in North Korea's history.
And as North Korea's capacity to do this nears, it narrows the options to prevent it, and heightens the risk of bad decisions. At stake in this stand-off are hundreds of thousands of lives on the Korean peninsula, and potentially across the entire region.