The United States hasn't invaded North Korea in over sixty years. Why does North Korea need nuclear weapons? The existing threat of artillery hitting Seoul is more than sufficient to prevent an invasion. We know this because the US hasn't invaded North Korea to prevent the development of nuclear weapons because of the more conventional threat to Seoul.
What is different about nuclear missiles? They can be used to attack countries other than South Korea. If North Korea shells or invades Seoul, they can't also deter an invasion by threatening to shell Seoul. They lose their deterrent if they use it. Meanwhile, nuclear weapons leaves them with two threats. So they can use one and retain the other. For example, they could invade South Korea while threatening other countries. Or they could use nukes on a more distant country while threatening Seoul if invaded.
If they wait for the right moment, they might be right. Barack Obama did not intervene militarily when Russia invaded Ukraine. He might not have countered a nuclear North Korea if it had advanced to Seoul. But the US might react differently to an attack from North Korea. For one thing, Obama is no longer president. Also, North Korea is not China much less Russia. It's a small country with few places to hide missile infrastructure and limited missiles. But even if wrong, they might still have to invade to find out they're wrong.
Given that, why does the US not just accept that, and attempt to mend relationships?
There is no evidence that North Korea wants a mended relationship. Obama was president for eight years. His biases favored diplomatic relations, which he opened with both Iran and Cuba. If North Korea wanted mended relations, they had eight years to develop them. And that ignores the fact that during the George W. Bush and Bill Clinton administrations, the US was actually giving North Korea aid. They weren't exactly hostile to diplomatic relations.
North Korea could have easily had a "mended" relationship for twenty-four years. While I wouldn't have followed Donald Trump's approach, as it is too noisy in my opinion, it's not much of an obstacle to normalized relations.
They could do something about it
when they can't do anything about it?
But that's the thing. In this particular case, the US could do something about it. The situation today is that North Korea is no particular threat to the US or even Japan. They are close to developing such a threat. But they are still at the point that a preemptive attack would work. However, that doesn't prevent a conventional artillery attack on Seoul.
It is generally acknowledged that if North Korea did use nuclear weapons on the US, the US would then destroy it. But if that's what's going to happen, the cheapest time for it to happen is now. North Korea will never be weaker than it is today. And Kim Jong-un is on a train that can only go two ways. One way leads to an invasion of South Korea. The other way leads to the fall of his government.
Any other options he might have are already available to him. If they interested him, he could have pursued them during the Obama administration. Instead, he pursued nuclear weapons, which are on the path to invade South Korea.
The question then is not if the US and North Korea will go to war. The question is when they will do so. Pretending that that is not the reality won't make it any less true. And delaying that war until later doesn't help the US position at all. The US is at its strongest relative to North Korea today. Delay only makes North Korea stronger without making the US stronger.
The only other real option is that China steps forward and removes North Korea's ability to produce nuclear weapons. However, they have shown no signs of being willing to do that. Perhaps the US could use trading relations to pressure China, but the truth is that China exports far more to the US than the US exports to China. That path leads to more US pain than Chinese pain.
- The US does nothing and North Korea becomes stronger.
- The US bribes North Korea with aid and North Korea agrees to stay where it is (and keeps to that agreement despite breaking previous agreements).
- China steps in. Unlikely, since they helped create the current situation.
- A military solution. Very painful for South Korea.
If #3 is not really on the table and #2 and #1 are unacceptable, what's that leave? From that standpoint, the US is just waiting for South Korea to realize that and evacuate Seoul.
There are no good solutions. They are only different types of bad. Obama tried #1. Bush and Clinton tried #2. Net result? We're here today. Trump is pursuing #3, which I find unlikely. Absent that, eventually either the US or North Korea will move to #4.