After the Korean war you Chinese don't want to try again to invade South Korea since it still exists. Even just roughly 20k American personnel is enough to deter any Chinese aggression as well as North Korean aggression. Same as invading Taiwan. Whats stopping the Chinese from invading it? Perhaps in response to the Korean war has shown America's will to intervene. And you can try to point out that we didn't intervene in Ukraine because we were scared. I could say the same for Russia not intervening in protecting Serbia from being bombed because they are scared.
OLD MAN you need to learn to read history books and use your brains. China didn't want to get involved in the Korean mess, but Kim went to Stalin for assistance. Stalin only agreed if China would provide assistance and Mao did gave Kim his support but the Russians already made it clear they will not send troops to fight the war. On the other hand PRC was planning to invade Taiwan but had to abort it because the US was helping South Korea and the North couldn't handle it. Instead going after Taiwan, the PLA went to help the North and we pushed you guys back until a stalemate was reached. The PLA and North Korea didn't have modern weapons back then and were under equipped therefore suffered high number of casualties. Even then you guys couldn't beat us, and your lousy general wanted to use atomic weapons against the ill equipped PLA and North Korea.
After the war has ended for a while North Korea did ask Mao for help in launching an invasion but this time China refused to help because the price would be too high and we rather focus on development.
As for the Yugoslavia, Yeltsin wasn't pleased the US and NATO went waging war against his ally but didn't do anything because 1 USSR had collapsed, Russian economy was in deep sh!t and Yeltsin was known to be PRO WEST.
Fast forward to 2008, this time the real Russian bear taught Georgia a lesson and NATO/US didn't dare mess with a stronger Russia.
Same with Syria when the US was so eager to bomb the country back to stone age Putin intervened, came up with a diplomatic solution and the world saw how Russia could prevent another senseless war waged by the Americans.
Last year North Korea conducted another nuclear test and the US's reaction was sending stealth bombers and preparing to strike their facilities. Now this time China warned the US not to cause trouble in our doorstep, news spread quickly PLA was heading to the borders and Kerry made the announcement that the US might have provoked North Korea LOL.
I do not have to tell you what happened recently regarding Crimea, as the world knows the US ain't going to war with Russia.
Make no mistake, the reason why China hasn't invaded Taiwan is because we are still developing and modernizing. We prefer peace and peaceful unification. If the student protests reflects Taiwan's future outlook then we can say Taiwan simply does not wish for joining the Mainland. We know the US will intervene, the US made that clear in 1996 during the Third Taiwan Strait Crisis. This is why we have modernized at a rapid pace and in the near future you guys won't want to repeat what Clinton did.