Since World War II, the US has sometimes used the pretext of humanitarian intervention in order to rationalize military actions which were actually (or were thought to be) in its own interests. But such pretexts have usually been the least significant part of the argument for war.
Historically, Americans have had little interest in what takes place in other countries, even in the countries with which they are connected through ancestry. Despite the mayhem taking place in Europe during the first two years of World War II, with the Germans overrunning most of the Continent and raining bombs on London, it was only the attack by the Japanese at Pearl Harbor that brought the US into the war. In later conflicts, such as Korea and Vietnam, the main reason for US involvement was to "stop Communism," not so much to save the victims of Communist aggression in those countries as to stop the Communists from going further and "taking over the world."
The more recent wars—since 1991, chiefly in the Middle East—have had a more openly-stated humanitarian rationale, sometimes bolstered by false information and exaggerated accounts. In 1991 we were told, for instance, that the Iraqis were committing atrocities against the population of Kuwait. Yet after the Iraqis were expelled from Kuwait, the US allowed Saddam Hussein to go on a rampage against his internal enemies, the Kurds and the Shi'a Iraqis. In 2003, part of the rationale for the new war was that Saddam was still killing and oppressing his own people and that "we will be welcomed as liberators" when we would invade Iraq, in spite of the fact that the US was bombing much of Baghdad into rubble.
The key to US involvement in the Middle East in these wars—and up through the present in Syria as well—has been largely self-interest but with a degree of genuine humanitarian concern for those people victimized by fanatical groups such as al-Qaeda and ISIS. But one still needs to be wary about announced rationales by any government involving idealistic motives for helping people. It's difficult for any private citizen to know how severe a given situation is internationally. Not only do government and corporate interests often lie to the public, but it's impossible to know whether the US might introduce more problems by intervening in a conflict. We are given selective information by the media, and often for months or years we're liable to hear nothing about a given part of the world where a crisis may have occurred or may still be going on.
A situation, however, where most people would agree that humanitarian intervention is justified and necessary is when an actual genocide is being committed. Yet the US did nothing to stop the Rwanda genocide of the 1990s. In the Balkans, the US did intervene in Kosovo to stop the killings but did not do so in the earlier period when the Bosnian Muslims were being massacred.
Unfortunately, the US is in a situation today where government seems even less inclined than ever to help people from outside the US. People from just south of the border have been systematically rejected and demonized by the present leadership of the US, which, under the cover of conservative values, is carrying out a form of nativism and racism in order to satisfy their political ambitions and the demands of their "base" in the US.