So North Korea has tested an intercontinental ballistic missile. That is definitely alarming, though a few caveats: the missile is almost certainly not operational with the North Korean armed forces, and nor have we seen evidence that North Korea could mount a minituarised nuclear warhead on such a missile. Moreover, there are reasons to doubt North Korean missile-guidance capabilities.
Still, North Korea is crossing a threshold which many have long feared it would, and this changes the deterrence dynamics not just of the Korean Peninsula but of North Asia. In future, when American presidents have to make decisions about war and peace in Korea, they don't only have to think about the damage a conflict would cause to the people of Seoul or Busan or even Tokyo, but also Alaska and in future the continental US. Granted, that concern was not entirely absent in the past. There's no rule saying a nuclear warhead has to be delivered on a missile - it could conceivably be smuggled to Washington, DC in a shipping container. But that's high-risk, and ballistic missiles still provide the most 'secure' means to deliver a warhead to a target.
US Administrations including this one have long said a North Korean ICBM is unacceptable, but unless the US is prepared to go to war, and risk millions of Korean and Japanese lives in the process, accept it Washington must. Arms Control expert Jeffrey Lewis has some bracing language about what this all means:
Accepting the unacceptable requires a collective effort from Washington to do the hardest thing imaginable: to admit that a policy has failed, that there is no five-paragraph op-ed that promises to produce a happy ending. North Korea is a nuclear-armed state and we have to deal with that. We have to admit that we have interests beyond denuclearization, and that those interests include deterring North Korea but also accepting that North Korea deters us. We have to talk with Tokyo and Seoul about living with a nuclear-armed neighbor. And we have to talk to Pyongyang about reducing tension.
On the question of talking with Tokyo and Seoul about living with a nuclear-armed neighbour, one wonders what President Trump meant with this:
What could South Korea and Japan possibly do at this point to reverse the situation? During the election campaign, Trump flirted with the idea of Japan having its own nuclear weapons, so perhaps that was on his mind. One of the reasons America is allied with South Korea and Japan is precisely to prevent nuclear proliferation – as long as Seoul and Tokyo can rely on US extended deterrence, the theory goes, they don’t need nuclear weapons of their own. A Japanese or South Korean nuclear capability would therefore signal a certain lack of trust in America's nuclear umbrella, though it wouldn't necessarily mean a complete breach. The UK , after all, is America's closest ally and relies on that nuclear umbrella to a degree, but it also has an independent nuclear deterrent.
Whether that model is applicable to South Korea or Japan is open to debate. But either way, a North Korean ICBM clearly erodes America's position in relation to its two North Asian allies. And as Jeffrey Lewis goes on to say in his article, it also weakens Washington's hand in the main Asia power contest with China. If you agree that Beijing ultimately aims to displace the US as the primary strategic power in the region, then a world in which Japan and South Korea have less trust in America's ability and willingness to defend them against North Korea is one that works to China's advantage.