Liberia, are we morally obligated?

Yes, the US has a moral obligation to help because of historical ties.

From NY times opinion
Leadership Needed on Liberia

All the elements of a desperately needed American intervention in Liberia are moving into place. Last night the United Nations authorized a multinational force, to be succeeded by blue-helmeted peacekeepers this fall. West African soldiers could take up positions in Liberia on Monday. The first two of three warships carrying 2,300 American troops arrive offshore this weekend. The only thing missing is an order from President Bush for those troops to go ashore and join the West Africans in trying to impose a cease-fire and stop the killing. While he hesitates, Liberians die.
Mr. Bush is not showing the decisive American leadership he likes to boast about. Understandably, people in Africa are drawing cynical conclusions. They wonder whether Washington's willingness to put even modest amounts of American military power behind American principles simply evaporates where Africa is concerned, even when the population is welcoming, local countries are doing their share and the American role would be limited. All military interventions have risks. But similar peacemaking efforts have been carried out successfully by the British in Sierra Leone and the French in Ivory Coast.
Charles Taylor, the Liberian president the West Africans are trying to nudge out of power, is an international pariah who has fueled violence in several neighboring countries. A tribunal backed by the United Nations has indicted him for war crimes in Sierra Leone. In Liberia, the civil wars Mr. Taylor has started or provoked have killed 200,000 people, 6 percent of the country's population. Scores more die each week a cease-fire is delayed.
In a BBC interview, Nigeria's president, Olusegun Obasanjo, whose country will provide the bulk of the West African force, likened the idea of leaving American troops offshore to telling someone whose house was burning that a fire engine had arrived but could not be put to use until the fire was out. America should take a more active role, alongside the West Africans, in quelling the violence in Liberia.