The Battle of Gettysburg, fought on July 1-3, 1863, was the turning point in the history of the United States of America. Had the Army of Northern Virginia under General Robert E. Lee prevailed in this battle, it is likely that the already demoralized Union Army would have collapsed and Lee would have captured Washington DC and possibly even Philadelphia. The South would have won the War, Lincoln would have been captured or killed and the country would have remained divided. The North had never really been that much interested in fighting this war, anyway.
The author, Louis Philippe d'Orleans, Comte de Paris, did not serve in the Battle of Gettysburg, but he did serve in other battles preceding it, especially in the 1862 Peninsular Campaign in South-Eastern Virginia, where the Union Army was defeated and forced to retreat.