Just as, by standing by and allowing the massacres to happen, we bear some responsibility for the atrocities in Rwanda. We, as a powerful democratic country, had the ability to act and refrained from doing so.
I used to share that belief, until I realized the logistical and political problems that would've inevitably arisen. The US invasion and occupation of Iraq helped me see that. What precedent would that establish, and how would the rest of the world view us?
It would've taken an invasion of Rwanda and subsequent occupation as a military police state in order to put an end to the violence and atrocities that occurred there, if indeed we could've acted in time. Because all that happened in what, a month? We would've had to have worked fast, with no time to second guess ourselves or weigh the geo-political implications. It would've been the embodiment of the SouthPark creator's "Team America: World Police". That's not a good look.
Then you take something like WWII as an example, which was mainly about defending America and protecting our allies, with saving the Jews coming in at a distant third on the priority list. By the time we mopped up the remaining German resistance and began clearing the concentration camps, many of the Jews had already been extirpated. We didn't exactly "save the day" as the song lyrics say.