On the topic of China:
"Everyone who's saying China would do something, I honestly don't think that's the case.
Back in 1950, the situation was very different. Mao Zedong had just driven the KMT off the mainland, and was trying to consolidate his power in Beijing. The Chinese were devastated from literally decades of war, and Mao was looking for a way to unite people under the PRC banner. The war in Korea provided the perfect opportunity. Additionally, the Chicoms looked over the border into North Korea, saw it collapsing, and saw the capitalist Americans behind the wheel. They got mighty nervous once the KMT's strongest backers got within invading distance of the newly-minted PRC, especially with an incredibly bellicose general in MacArthur at the head of the offensive. Additionally, the PRC wasn't the economic powerhouse it is today, and it had no formal relations with the United States, and regarded it as little more than an imperialist aggressor. The impetus to attack was there.
Today, things are much more delicate for the PRC. The old animosity they had with the United States is long gone, replaced by dollar signs in their eyes from all the massive bilateral trade agreements. Sure, they're trying to ape our military tech and counter our might, but they're too invested in us, and we in them, to make that move. Additionally, they've got lucrative trade deals in place with the South Koreans, Japanese and the various constituents of the European Union, as well; all of which are potential allies in a war with North Korea. The Chinese wouldn't just be facing half the civilized world in combat if they defended the North, they'd lose so much capital it would send them spiraling into an economic collapse."
-Anon from /k, they know their shit. MacArthur quite literally wanted to destroy China and nuke everything. America doesn't want to fight China, nor vice versa. No one really wants to take over North Korea because it'll be so costly, and if the North Koreans do anything rash the People's gargantuan army will rush in and take over, setting up a new government that isn't so crazy. And people would be ok with this, for this would be an awesome outcome. The North Koreans cannot take South Korea. If they attempted to, it'd be at massive loss of life to the North Koreans - and this would be the greatest tragedy.