Having just watched “They Shall Not Grow Old”, and listening to the testimonies of the men who fought WWI, what struck me was how nonchalant they were about the whole thing, it was basically just a job to do.
The Brits and others, when WW I began, believed that it would be not only a short one, but one waged in a way that was not "modern". Germany had developed tactics and "stuff" that had never been used before, so things didn't turn out to be what the other combatants had supposed.
Airplanes were still very new, as were tanks...the Brits were still heavily relying on horses.
And then, there was the mustard gas. :-(