It probably goes without saying that one can't imagine a Roman Emperor apologizing for failing to prevent a genocide the way President Clinton has apologized for not taking action in Rwanda, but is that notion, as a general concept, primarily a reaction to the exposure of the average person to images of, say, the Holocaust?
It's worth noting that the phrase 'international policeman' itself is not terribly new and is not tied to humanitarian agendas. Notably, at the height of its post-Napoleonic power - ie, between Vienna and Crimea - Russia was explicitly nicknamed the 'gendarme of Europe'. Of course, Russian armies were used to suppress revolts as part of a programme of reaction but there's little new about the concept of 'world police'.
That's just a nugget that I can contribute, unfortunately. I can't speak with authority on the history of 'humanitarianism'