When has the US ever done peace keeping?
Like seriously. Taiwan is the closest thing i can think, and it is basically propping up a local government who is more friendly to the US than the alternative. Everywhere the US goes it promotes US power, sometimes that means keeping peace between two different governments which claim the same thing (like the People's Republic of Chine and the Republic of China) Other times it means punishing someone who dis-obeyed order (Saddam Hussein was a great friend of the US, invaded Iran for you guys, until he decided his just reward was Kuwait, and then there was war... cause an ally who wouldn't do what they are told isn't a useful thing, as anyone who plays diplomacy knows.)
I'm not saying Russia is any better, they have shown a willingness to project power into their neighbourhood and equally 'claim' in their propaganda that they were just keeping the peace. Crimea totally chose to return to Russian sovereignty. And Eastern Ukraine is just annoyed with the Fascists in western Ukraine (which may well be true, i don't particularly trust the EU's propaganda on this either, but the pro-Russian Ukrainians would likely not have started a civil war if Russia hadn't been will to give them arms and i suspect even troops.)
China has been in a two hundred year slump, it began falling behind in terms of gdp when europe and america was industrializing, but China has a long memory, over 2000 years long. So it is willing to take another 100 years to grow peacefully and not piss of anyone until it is ready to actually beat them.
Don't kid yourselves. Vietnam had every reason to go and support the communist ideology, they had suffered under French Imperialism for years, and after throwing the French out the US turned up... It is no surprise that they adopted the only anti-US and anti-Imperialist ideology going... and guess what, after beating the French, they beat the US, and then they beat the Khmer Rouge and the one of the first 'developing' nations to pull itself out of poverty...
In short, Vietnam was right to defend itself against the harmful ideology which had done so much damage (Imperialism) and the US acted like an Imperialist power by trying to subdue Vietnam.
Never the peacekeeper, always the Imperialist.