As Ulysses S. Grant and William Tecumseh Sherman prepared their inexperienced troops for a massive offensive by an equally green Confederate army in April 1862, the outcome of the Civil War was still very much in doubt. For two of the most chaotic and ravaging days of the War, the Union forces counterattacked and fended off the Rebels. Losses were great--more than 20,000 casualties out of 100,000 Union and Confederate troops. But out of the struggle,...