Why would the US want a peace treaty in the Korean peninsula?

Question

The United States have some projects in South Korea that would be negatively impacted by cooperation with the North:

• A ballistic missile defence (BMD) system in South Korea. Its defence area allow to use it against Russia and China as well. It is an important part of U.S. "global first strike" concept. Those BMD in Europe, Korea and other US allies should intercept possible return strike. They are (mostly) placed near opponent's borders to intercept missiles on their launch, which is much more simple, than intercepting near US borders.
• Natural gas supplies to South Korea. There is a project of building trans-Korean gas pipeline from Russia. And now, when Russia-China pipeline is build, this project is much more easy to build. Obviously, this will end US supplies to South Korea, because pipeline gas is much more cheap than naval-supplied. US is trying to prevent European situation where russian natural gas pipeline easily beats US naval supplies.

These are only available if there are tensions between the Koreas.

But is there something good, some advantages for the U.S.? I think that the U.S. could block negotiations (for example, starting military exercises, which are definitely provoke North Korea to start missile launches) if there were no advantages. But negotiations took place. So, what are the advantages to the U.S.? I have thought about it, but cannot imagine any.

I am aware of Why does the US engage with North Korea? My question is not why the US engages with North Korea on some level. My question is why the US does not attempt to prevent relations between the North and South. What advantages are there that outweigh the disadvantages that I have listed?

Answer 1

... why the US does not attempt to prevent relations between the North and South. What advantages are there that outweigh the disadvantages that I have listed?

China.

China is the greatest factor in the East/South-East Asia for the USA. China turned out to be shrewder than what USA thought it to be. With a vast reserve of liquid money, 1.5 billion people, and strong industrial base, China has become a real threat to the US hegemony.

In the pre-WW2 world, hegemony used to have been achieved and maintained through various means

• 8th century BC - 14th century AD : military might.
• 15th–19th centuries : discovery of sea routes, credit & money, control over sea routes, industry and rail road.
• 20th century and onward : economy and military technology.

In the post WW2 world, USA is the world's only hegemon. Militarily and technologically only Russia could displace the USA. But, Russia is continuously failing because of its direct confrontation. To me, Russia also maintains a faulty foreign policy as they consider international relations as a "Zero-Sum Game" which is a Cold War era mentality. As a result, although Russia has its petro-money, times and again Russian economy saw downward spirals. As all of us know, a country can't do much when the economy is not on the right track.

Contrary to Russia's bellicose attitude, China adopted a very shrewd policy. They maintained productive international relations through which they rapidly expanded their economy, quietly built their military, and, now they possess a vast reserve of liquid money. These elements are giving China various advantages:

• They are not losing their existing markets while going after new markets.
• Rapidly closing the technological gap with the West.
• Through Belt & Road initiative, quietly placing themselves in the control of other countries' economies.

China has taken the game into a completely new level where the USA's Cold War era tactics can't work. In order to take on China, USA needs stronger cooperation with its Asian allies and peace in the Korean peninsula as this will give the USA an option to engage all of its Asian resources to solve only one problem.

A ballistic missile defense (BMD) system in South Korea. Its defense area allow to use it against Russia and China as well.

This is not a very important aspect of the entire scenario.

A BMD in Korean peninsula will just make matters worse. Given enough time, China can also build their indigenous BMD system.

Natural gas supplies to South Korea. There is a project of building trans-Korean gas pipeline from Russia. And now, when Russia-China pipeline is build, this project is much more easy to build. Obviously, this will end US supplies to South Korea, because pipeline gas is much more cheap than naval-supplied.

This is a similar case with Europe where the USA intended to block a Russian pipeline (Nord Stream-2) to Germany^{1, 2}. Apparently Germany moved ahead with the project, but the tension is still on³.

I think, the USA has enough confidence that they can put diplomatic pressure on Koreans to keep them out of those kind of pipeline deals.