One thing I do want to make clear: I don't think the United States is an angel by any means.
And in fact, I think the United States pursues its self-interest quite vigorously.
However, I think that the way the United States has defined its self interest has been done in a way in which other countries happen to benefit.
Let me give you an example: During the European imperial age, the belief was that nations needed colonies to supply raw goods. Getting colonies required conquest, subjugation, etc. Not nice stuff.
The US, by contrast, defines its self interest by having a stable world order and prosperous countries to trade with. It doesn't *need* raw materials (and despite those uneducated enough to think we'd conquer a nation for its oil, oil is not worth conquering a country for, it's not valuable enough). So its policies have tended to be able ensuring global stability and creating markets for its products and services.
Sometimes that has meant supporting dictators and other goons who provide "stability". But usually it has meant the US defending countries being assaulted by others and trying to increase the overall propserity of the world.
In other words, the US strategy has been in its own self interest overall but its self interest has always been of benefit to the others. What is good for the US, as a generalization, has also been good for the world.
Since 9/11, the United States has recognized clearly that backing dictators because of stability is not the way to go. That the "freedom deficit" is coming back to bite us. The rise of Al Qaeda and other movements can be traced to this freedom deficit.
Which brings me to the irony: The same people who oppose the United STates having liberated Iraq from a monstrous dictator and attempting to create a democracy are the same people who complain that the United States supported dictators in the past. Bush would be the first to say "Yea, it's wrong to support dictators. We need to start vigorously promoting freedom instead."