>..but as a superpower, don't you think the US has some responsibility to stop genocide? Not saying that's the sole reason we did it..but my point is that there doesn't necessarily have to be a threat to the US to justify an invasion in some cases yes?

Good question.
Here's one way to look at it...
Let's suppose that we didn't have hundreds of thousands of troops waiting around to be sent in, and we had to initiate a draft to have the needed troops - as we did in WWII, Korea and Vietnam.
As a member of congress, would you have voted to initiate a draft to send troops into Iraq?