In 1863, Robert E Lee once again tried to bring war onto northern soil in Pennsylvania in an attempt to capture the capital. The battle lasted three days and was the bloodiest battle of the war (51,112 casualties). The Confederacy lost again and this became the turning point in the war as the Union gained momentum and began to weaken the southern army.
After the battle of Antietam President Lincoln tried to get the South to surrender and rejoin the Union. The South did not surrender so he freed all slaves in the Confederacy. The Civil War was officially about secession and slavery. The South lost the support of other countries b/c they refused to stop slavery.