The South was built on slavery yes, but the US is more than the South, and the Civil War the one of the first examples ever of a terribly destructive war being fought to liberate someone ELSE, something the US as a whole would do later in Europe, Korea, and tried to continue doing with much less success in Vietnam, Iraq, etc.

Which other culture ever fought wars to help other people, on moral principle (besides Revolutionary France)...?

It is the same Judeo-Christian morality that forms the basis for the SJWs today, fighting for other groups at the expense of their own. Not something many other cultures have done. One example might be the Vietnamese intervening in Cambodia to stop the Khmer Rouge...