Suicidesoldier#1
Sure, if they're needed.
But you're better off creating a super entity that watches over all of mankind that is apolitical and not of any specific country. : P
In theory, this is what the UN is for really. They have their own forces, constructed of troops from around the world and should act as a world police. I suppose the issue is that there's a lot of politics involved in this, as each country wants to impress their own views and policies into the UN actions, so becoming impartial is sort of impossible here.
As such, its not a perfect system, and it does tend to be undermined when countries take outside intervention (Iraq, Afghanistan, Vietnam to name the first ones that spring to mind). Being an optimist on the situation, I do believe that if countries could make better use of the UN framework, it would reduce the perceived need of certain powers to intervene in the disputes of other nations.