The U.S. Civil War gunboat Cairo is shown in this undated photo. The Union gunboat was sunk on Dec. 12, 1862 while clearing Confederate batteries and obstructions on the Yazoo River, north of Vicksburg, Miss. The Cairo was sunk by explosive charges from detonated electrical mines. (AP Photo)

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In this photo provided by the Library of Congress, a group of men pause for a photograph during the Battle of Chattanooga, September-November 1863, in Chattanooga, Tenn., during the Civil War. (AP Photo/Library of Congress)

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In this photo provided by the Library of Congress, the camp of the Tennessee Colored Battery is pictured during the Siege of Vicksburg at Johnsonville, Tenn., in 1864., during the Civil War (AP Photo/Library of Congress)

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In this photo provided by the Library of Congress, Gen. William T. Sherman, leaning on breach of gun, and staff at Federal Fort No. 7 in Atlanta, Ga., in September 1864. (AP Photo/Library of Congress/George N. Barnard)

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In this photo provided by the Library of Congress, African Americans prepare cotton for the gin on Smith's plantation, Port Royal Island, S.C., in 1862. (AP Photo/Library of Congress/Timothy H. O'Sullivan)

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In this photo provided by the Library of Congress, a group of African Americans on J.J. Smith's plantation, Beaufort, S.C., pose for a photograph in 1862. (AP Photo/Library of Congress/Timothy H. O'Sullivan)

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Civil War camp life.(AP PHOTO/LIBRARY OF CONGRESS)

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In this photo provided by the Library of Congress, Gen. George Custer stands for a portrait some time between 1860 to 1865. Location is unknown. (AP Photo/Library of Congress)

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In this photo provided by the Library of Congress, rear view of the camp of Captain John J. Hoff of the Union Army in Gettysburg, Pa., in July 1865. (AP Photo/Library of Congress/William Morris Smith)

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This 1865 photograph shows the entrenchments occupied by enlisted men and officers at Fort Sedgwick, Va., during the American Civil War. Heavy artillery duels took place at Fort Sedgwick, also known as Fort Hell. The bomb-proof living quarters are constructed from earth and timber with wooden barrels projecting as chimneys made of wood and mud in place of brick. (AP Photo)

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In this photo provided by the Library of Congress, Union soldiers rest in trenches before the Siege of Petersburg in Petersburg, Va., Dec. 1864, during the Civil War. (AP Photo/Library of Congress)

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In this photo provided by the Library of Congress, Confederate prisoners captured at the Battle of Fisher's Hill, Va., in September 1864, sent to the rear under guard of Union troops. (AP Photo/Library of Congress)

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In this photo provided by the Library of Congress, the guard mount of 114th Pennsylvania Infantry (1st Division, 3rd Corps), in Brandy Station, Va., in March 1864. (AP Photo/Library of Congress/Timothy H. O'Sullivan)

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In this photo provided by the Library of Congress, President Abraham Lincoln's Gettysburg Address, in Gettysburg, Pa., on Nov. 19, 1863. (AP Photo/Library of Congress)

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In this photo provided by the Library of Congress, unidentified men sit at a table at the noncommissioned officers' mess of Company D, 93rd New York Infantry, Union Army, in Beatleton, Va., August 1863, during the Civil War. (AP Photo/Library of Congress/Timothy H. O'Sullivan)

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In this photo provided by the Library of Congress, the Drum Corps of the 93rd New York Infantry in Bealeton, Va., in August 1863. (AP Photo/Library of Congress/Timothy H. O'Sullivan)

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Allan Pinkerton, Battle of Antietam, 1862.(AP Photo/Library of Congress)

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In this photo provided by the Library of Congress, the 150th Pennsylvania Infantry camp on Belle Plain, Va., is pictured in March 1862, three weeks before the Battle of Chancellorsville, Civil War. (AP Photo/Library of Congress)

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