Were some of the men very brave? Sure. But it's not like they had much of a choice but to be anything other than what others who weren't there would call "brave", what else were the men in those positions suppose to do if not just keep going and try to live? Let themselves be captured, tortured and brutally murdered? Just allow themselves to be shot and killed in the field? Kill themselves on purpose? Or, what many of them did, just keep going and try their best?
Also, just so you know, Germans never did call Marines "Devil Dogs", it was a recruiting campaign by US Marines and civilians, never had anything to do with the Germans of WW1, they never had a unique nickname for any of their enemies. This can easily be researched yourself, and even the Marine Corps own History Division says the same. On the original posters and prints that had it, it wasn't even correct German either.