While most of the major battles of the Civil War were fought in Confederate territory, the battles of Antietam and Gettysburg took place inside the Union. Both were the result of Confederate General Robert E. Lee's attempt to reduce the pressure on the Confederate capital by striking north. Neither battle proved successful for the Confederacy, and both were typified by extremely high casualty counts.
The Battle of Antietam took place on Sept. 17, 1862, between forces under General Lee and General George McClellan. It took place near Sharpsburg, Md, and involved the Confederate force attacking a much greater Union army. Fortunately for Lee, however, the commander of the Union forces was a general famous for his inability to take decisive action, and despite having nearly twice as many men, McClellan missed a golden opportunity to destroy Lee's army and cripple the Confederate war effort.
The Battle of Gettysburg took place on July 1-3, 1863 at the town of Gettysburg, Pa. Lee had once again struck north, and this time the balance of forces was much closer to equal. Lee attempted to concentrate his forces and destroy the Union army under George Meade and John Reynolds, but a series of decisive assaults on the Union lines failed, and reinforcements arrived in time to force a Confederate retreat.