Yeah, that's absolutely true, and I can't deny that his principal role should be to look after American interests and be accountable to the American people.

Nonetheless, there is

1. scope for overlap between American interests and the welfare of people in the Third World. Even from a business perspective, the more healthy and wealthy people in stable countries there are, the more consumers there are of US goods, and the more profitable US companies are. In relation specifically to Israel and Palestine, if an American president (the only person both parties will listen to by reason of America's strength) does not keep a lid on things, it will boil over into the region and harm US busioness interests, the global economy, and US access to Middle Eastern oil.

2. a moral imperative (not an obligation, but an empathic urge), to stop suffering in other countries. Spread the benefits of democracy, rule of law, the rights of the individual. Its working in Afghanistan. That has a flow-on affect for US security - if people are well and happy, then they'll be less likely to try and drop planes into buildings. Discontent breeds violence. Keep people content = less likelihood of violence.