It's rare for a modern writer to make a genuinely new discovery about the Civil War, but former
intelligence officer Edwin C. Fishel pulls it off in
The Secret War for the Union. Having stumbled upon a large collection of previously unknown documents at the National Archives, he describes in this book the undercover operations of the
Army of the Potomac. Federal intelligence, by Fishel's account, was crucially important to winning the war, and was of much higher quality than previously assumed. Among other accomplishments, it appears to have played a vital role in the Union victory at Gettysburg. This surprise--and a few others--await serious readers.
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Background Information
The main Union force protesting the national capital and the Eastern states during the Civil War was the Army of the Potomac, which under generals Meade and Grant eventually brought the war to a close.