as the fighting begins, Henry forgets his fears. in what way does he experience a sense of brotherhood?

The fight is about to begin, and before he is ready or can consciously decide, he lowers his rifle and fires the first shot of the battle. Henry then loses concern for himself, and becomes "not a man but a member." Whatever he was part of, it was in a critical state, and he was part of its desire. The noise of the firing reassures Henry in his actions.