Partly because MacArthur was looking (and talking) like he was going to press on into China as he stormed up the peninsula after the landings at Incheon.

It's also worth noting that 1950 China has very little in common with 2011 China.

Indeed, but it also has some factors in common through different pathways - I grow nervous when I read of the ultranationalist riots permitted by the Party, because it seems like the kind of thing that would itself to the leaders deciding to enter 'a short, victorious war'. (Defending the Middle Kingdom, defending the Revolution; rather similar.)

Entirely different, see my comment above about Deng Xiaopeng's ascendance in the late 1970s and the quite severe repercussions for the perpetrators of the Cultural Revolution. There's no linkage between the revolution and nationalism anymore. "A cat that catches mice is a good cat, whether it's white or black".

Nationalist pride, sure, that's been a constant through any cohesive nation's history. Koreans aren't Chinese, though, and the leaders of China are too smart to look for a "short, victorious war" on the Korean peninsula..

I didn't say nationalism and the Revolution were the same thing, I said they could play the same role.

Mao's China was far more brutal and crazy than modern China, and 1950 North Korea was to some degree less brutal and crazy than modern North Korea. In 1950 they were peas in a pod. Since then, they have grown in different directions.

China wasn't a growing economic powerhouse in 1950. In fact, it didn't have much of an economy, period. They've made too much progress, established too many economic links, gained too much international influence to throw it away on a stupid war that, even in "victory", would be a loss.

They don't even have an idealogical interest -- we throw around the word "communist", but North Korea's modern ideologies are so far removed from Chinese communism that it's practically apples and oranges.

And the Cold War is long over, nobody's out to "eradicate" or even "contain" communism. US-Chinese relations are strained, but not hostile, and they know we wouldn't be looking to cross the Yalu.

Even more than the macro factors, the 1950s Chinese state was a revolutionary state bent on the worldwide spread of communism.

In the late 1970s, Deng Xiaopeng killed off the last group of leaders trying to keep that sentiment alive (the Gang of Four) and since then they've been all pragmatism, all the time. Any talk of communism is window-dressing, their regime is ideologically committed to doing what's practical.

So even aside from the cost-benefit, they have no ideological incentive to throw their lot in with the North Korean gov't. As I said upthread, the reason they give them aid right now is because a refugee crisis in Manchuria would be inconvenient. Pragmatism.