Red Indian, you're absolutely correct about the US in regards both slavery and the "Indian wars" (mostly massacres) of the 19th century.
Vietnam is a hair less clear-cut...still not real good mind you, and definately debatable.
To more directly answer Qjakal on why her version of world gov't is still a bad idea:
Different parts of the world are at different states of cultural development. That's something the "politically correct" crowd doesn't like talking about, but it's true. It's one reason a 3rd world nation such as Mexico with strict gun control can have a murder rate triple the US rate, while a very culturally advanced nation such as Switzerland can literally have a machine gun in every home yet have a murder rate only a small fraction of the US rate. Murder rates aren't connected to gun access, it's connected to cultural violence.
For a horrifying example of current cultural violence problems, go do a websearch in
www.google.com for the term "honor killings" (with quotemarks so it looks for those two words in sequence. Fact: the #1 cause of death for Palestinian women is being killed by family for supposed "sexual dishonor" - and Jordanian law (which Palestinian law is based on) condones the practice. "Honor killings" occur all over the middle eastern world, all the way out to India.
This is just one example of cultural-level problems. (And before you ask: no, this is NOT an issue with Islam, "honor killings" as practiced across the Arab world are in conflict with the Quran, it's a *cultural* thing.)
A bigger problem is that the basic idea of government being legitimate based only on the active will of the governed is an alien concept for the MAJORITY of the human race. Most people today are governed by whoever was able to scrape together the most brute force.
Those people are NOT ready to be able to boss me around.
I won't allow it.