Where was the artilleryman when he had his vision?

In Walt Whitman's poem “The Artilleryman’s Vision," the title character has a vision or a flashback about the time he spent in the war; essentially, he reflects about the experiences he had as a soldier and an artilleryman. It is not revealed which war the artilleryman refers to, but many assume that the vision he has is about the Civil War. As far as his whereabouts are concerned, the artilleryman has his vision while he is at home, in his bedroom, as he lies on his bed next to his wife. His child is in the room next to them. This is how we know that the artilleryman's vision is actually a memory, as he basically relives it in his mind.