Lets be honest with ourselves here: The US is not a philantropic organization.
When they intervene across the world stage, it is with their interests in mind first and foremost. Sometimes, doing the right thing happens to coincide with this and sometimes, it doesn't.
The vast majority of the US intervention in the ME was to keep the oil flow stable.
The US' direct involvement in WWII was the result of aggression from the Japs.
The US fighting the cold war was about keeping communism (a perceived evil, I won't go into that debate) as far away from their borders as possible.