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.