That was because Kirk Douglas wanted to be the flawless hero and Kubrick wasn't big enough to push back. When they got back together to make Spartacus three years later, they butted heads so much Kirk told him, "This ain't your show" and Kubrick called him a kike because that's what he was.
But for real brotha man that trench attack was one of the best battle scenes ever. We don't get many WWI movies because it wasn't a big part of American history.