David Brooks is an op-ed columnist for the NYT.

Tuesday is the 150th anniversary of the second day of the Battle of Gettysburg. In his eloquent new account, “Gettysburg: the Last Invasion,” the historian Allen Guelzo describes the psychology of the fighters on that day.

A battlefield is “the lonesomest place which men share together,” a soldier once observed. At Gettysburg, the men were sometimes isolated within the rolling clouds of gun smoke and unnerved by what Guelzo calls “the weird harmonic ring of bullets striking fixed bayonets.” They were often terrified, of course, sometimes losing bladder and bowel control. (Aristophanes once called battle “the terrible one, the tough one, the one upon the legs.”)

But, as Guelzo notes, the Civil War was fought with “an amateurism of spirit and an innocence of intent, which would be touching if that same amateurism had not also contrived to make it so bloody.”...