The Killer Angels by Michael Shaara

On June 15 the first troops of the Army of Northern Virginia, Robert E. Lee commanding, slip across the Potomac at Williamsport and begin the invasion of the North.

It is an army of seventy thousand men. They are rebels and volunteers. They are mostly unpaid and usually self-equipped. It is an army of remarkable unity, fighting for disunion. It is Anglo-Saxon and Protestant. Though there are many men who cannot read or write, they all speak English. They share common customs and a common faith and they have been consistently victorious against superior members. They have as solid a faith in their leader as any veteran army that ever marched. They move slowly north behind the Blue Ridge, using the mountains to screen their movements. Their main objective is to draw the Union Army out into the open where it can be destroyed. By the end of the month they are closing on Harrisburg, having spread panic and rage and despair through the North.