Although the US did not enter World War II in response to German genocide against the Jews and others, we collectively promised ourselves then to never again permit a holocaust on so grand a scale. There are other places in the world where the massacre of civilians occurs of far grander scales. The Soviets and Communist Chinese have annihilated millions in this century. The Khmer Rouge systematically killed millions in Cambodia. More recently, innocents have been ruthlessly slaughtered on a large scale in the Congo and Rwanda.
Slavery still exists in the Sudan.
Atrocities in Sierra Leone cry out for amelioration. The People's Republic of China still suppresses Tibet. Clearly, if the US wishes to police the world on humanitarian grounds, there are more problems than we have resources to solve. The inability to resolve all problems, however, does not relieve us of the responsibility to act when we can. One way to grapple with this dilemma is to limit intervention to those situations where there is a significant overlap between humanitarian concerns and US national interests.