Genocide happens every single day in a ton of countries and we do nothing. Its not where we draw the line. In the old days, maybe. But now? Now at most we puff out our chests and threaten "pin prick" bombings.
We do nothing because we have other things we consider more important. The US is coming to the idea that we cant be the worlds police. We don't try and stop all the little genocide, but we tend to jump in during overt large scale genocide. Again, not because we are super nice people, but because its in our interests.
When I said that the US is letting them fight it out but stopping at nukes and genocide, I meant that the US is going to do nothing up until those 2 things. With nukes, when we jump in, it will be a strong reaction. With genocide, it would start out with a "noooo guys, come on, dont do that". If the world cries thought, we will jump in with some force. Either way, the US would prefer no genocide because its more for us to deal with.
