Warfare in the eastern theater began poorly for the Union. U.S. General George B. McClellan failed to capture the Confederate capital of Richmond, Virginia in his Peninsula Campaign and retreated after attacks from Confederate General Robert E. Lee. Meanwhile, both sides concentrated in 1861–1862 on raising and training new armies. The main action was Union success in controlling the border states, with Confederates largely driven out of border states. The autumn 1862 Confederate retreat at the Battle of Antietam led to and Lincoln's warning he would issue an Emancipation Proclamation in January 1863 if the states did not return. Making slavery a central war goal energized Republicans in the North, as well as their enemies, the anti-war Copperhead Democrats and ended the chance of British and French intervention. Lee's smaller army won battles in late 1862 and Spring 1863, but he pushed too hard and ignored the Union threat in the west. Lee invaded Pennsylvania in search of supplies and to cause war-weariness in the North. In perhaps the turning point of the war, Lee's army was badly beaten at the July 1863 Battle of Gettysburg and barely made it back to Virginia. In July 1863, Union forces under General Ulysses S. Grant gained control of the Mississippi River at the Battle of Vicksburg, thereby splitting the Confederacy. In 1864, Union General William Tecumseh Sherman marched south from Chattanooga to capture Atlanta, a decisive victory that ended war jitters among Republicans in the North hand helped Lincoln win re-election.