Why can 1863 be considered the turning point of the Civil War?
1863 was a very pivotal year in the Civil War. Staring January 1 of that year, the Emancipation Proclamation went into effect. This move freed the slaves in the "states in rebellion." Lincoln was able to give the war a higher moral cause and still claim authority over the seceded states at the same time. The North also instituted a draft this year. New York City experienced draft riots in the summer of 1863 which were put down only with the use of soldiers.
1863 was also very important from a military perspective. In May of that year, Lee's forces defeated the Union army at Chancellorsville but Lee's best general, Thomas "Stonewall" Jackson, was killed in a friendly fire incident. Lee would later go on to divide his army into three corps and invade the North. At the Battle of Gettysburg, Lee's army was defeated by Union forces under General Meade. Lee would never invade the North again, and most historians see the Battle of Gettysburg as the beginning of the end of Southern independence. In the West, Grant was able to win the siege of Vicksburg which was the Confederacy's last stronghold on the Mississippi River. Grant would later go on to be the overall commander of the Union armies thanks to his determined nature. The military failures combined with the Emancipation Proclamation ruined any hopes that the Confederacy might have had in gaining European support for their cause.
1863 can be seen as the turning point of the Civil War in both military and political terms.
Militarily, this was the year of the battles at Gettysburg and Vicksburg. Gettysburg broke a Confederate invasion of the North. Vicksburg gave the North control of the Mississippi River and cut the South in half. Both were major turning points in the war.
Politically, this was the year the Emancipation Proclamation took effect. This made it so the war was a war about slavery, not just about keeping the Union together. This gave the North the moral high ground in international public opinion and helped ensure that European countries would not recognize the Confederacy.