Henry is encountering the experience of actually being involved in fighting, shooting his weapon at the enemy. After all the doubts and fears and uncertainty about how he would react to being in a battle, the time has come and he no longer has time to consider what he is doing. His actions are based not on conscious choice or planning that he has thought out for himself. Instead, he is being swept into the action of the moment - and is discovering that he deeply cares about the others in his group, about their safety, about the flag that is the symbol of their unity and pride. He has not yet been injured, which he sees as being the proof of being "a man", but he demonstrates by his actions that he has become a member of the army and part of the cause for which it is fighting.
He suddenly lost concern for himself, and forgot to look at a menacing fate. He became not a man but a member. He felt that something of which he was a part - a regiment, an army, a cause, or a country - was in crisis. He was welded into a common personality which was dominated by a single desire.