This is actually a rather large can of worms.
Allied propaganda is one thing.
The two are not very often synonymous. I'm not suggesting that German soldiers committed NO misdeeds in Belgium and France, for those who are prone to leaping to the opposite polar statement. The concept of a humane, or civilized war predates WWI by quite a lot. But, even prior to this, there are lots of examples of where implicit or traditional 'rules' existed. The concept of Civilized War and its dark-side counter part, Brutal War live side by side.