The correct answer to this open question is the following.
Although you did not include options, we can answer the following.
According to McClure, the battle was the most decisive for the future of the Confederacy was the Battle of Gettysburg.
During the times of the American Civil War, one of the bloodiest battles between Confederates and the Union Army was the Battle of Gettysburg, fought on July, 1, 2, and 3, 1863. It is considered a decisive battle in the future of the Civil War in that the Union Army won the bloody battle. Union General George Meade led the Army of the Potomac and defeated the Confederated troops led by General Robert. E. Lee, in Gettysburg, Virginia. The Union had stopped the imminent Confederate Army invasion of the North, which represented a major turning point in the Confederate aspirations.