On September 17, 1861, Union general George McClellan and Confederate general Robert E. Lee fought to prevent the Confederates from moving on Maryland. There were 23,000 casualties, but the battle was inconclusive.
Union general Ulysses S. grant attacked the Confederates at Vicksburg on May 18, 1863. They trapped the Confederates there until June 4, when the Confederates surrendered.
On July 1st of 1863, Confederate Robert E. Lee attacked Gettysburg. The Union fought for 3 days before the Confederacy was defeated. There was 160,000 soldiers, and 51,000 casualties--more than any other battle.