If I recall My History...

Right now it is fairly hard to turn on any news channel or check a news website and not have something about the possibility of attacking Syria thrown in your face, at least if you live in the US and/or are checking a US news site. A lot of it is one side saying we should because there is a moral imperative, while the other says the American people do not want to be involved. If I remember what I learned in school correctly, that other side is much more correct than the side claiming a moral need.
If you consider what has happened int he world during the history of the United States, you will find numerous examples of genocide and genocide-like tragedies that we did nothing about. The reasons for that largely stem from our disinterest in intervening in another countries affairs, outside of normal political operations. Even when we know about the genocide, we want to leave it alone and be left alone. Look at modern North Korea, where people are starving under they tyrannical, communist regime. Look in modern Egypt where Christians are being murdered and churches burned. I'm sure there are many other nations in Africa that are also suffering similar tragedies, but I cannot name them. Look to the past though and you can find even more genocide and genocide-like tragedies that we did nothing about. Stalin of the USSR starved millions of Ukrainians to death during the Holodomor, and we did nothing. Even the Holocaust under Hitler did not lead us to take action, and we did know some of what was going on there. That is part of the reason why, at the time, many people did want to go to war, to stop Hitler and what he would do to Europe, but they knew the American people detest war, so it took the war coming to us to ultimately force any direct action.
You can take the point I am trying to make one of two ways. Either America does not have an interventionist history, because the people did not want it then and do not want it now, or that if we are to claim the existence of a morale obligation, we have a lot of work to do to satisfy it.