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?
Exactly. The point isn't that we should always intervene, it's that we bear the moral responsibility for our actions and for our refusal to act. The standard confession in the Episcopal church asks forgiveness "for what we have done and what we have left undone."
Rwanda was a hard case. There was no local support for our intervention; we had no insight into the culture or history of the locals; there were no economic, historic, or cultural ties that would give us an entree; there was no stable internal group or power center that would align with our intervention. Yet, on the other side, the atrocities were appalling.
Truth is, I think we couldn't have intervened effectively and did right to stay out. But it's a lesser-of-two-evils "right."