A lot of times when the US is involved abroad, such as during a war or supporting a rebel group or whatever, people argue about what the reason is, like there has to be just one.

I contend that while there may be exceptions, America moves when realpolitik reasons and ideology combine.

Realpolitik reasons are things like Oil, power, hemming in other nations, keeping canals open to us and stuff like that.

Ideological reasons are things like spreading democracy, protecting our way of life, human rights, and so on.

With just one or the other the US won't tend to move. Just doing something for realpolitik reasons usually hurts bad politically as the people won't stand for it. And those with the countries interests in mind (and often their own) will be reluctant to just do something for an ideological reason.

I guess what I'm saying is that when arguing why the US does something you're at least looking for two reasons.