From Wikipedia

The Civil War
Episode Nine

The Better Angels Of Our Nature

Historians have debated whether the Confederacy could have won the war. Most scholars emphasize that the Union held an insurmountable long-term advantage over the Confederacy in terms of industrial strength and population. Confederate actions, they argue, only delayed defeat. Southern historian Shelby Foote expressed this view succinctly: "I think that the North fought that war with one hand behind its back...If there had been more Southern victories, and a lot more, the North simply would have brought that other hand out from behind its back. I don't think the South ever had a chance to win that War." The Confederacy sought to win independence by out-lasting Lincoln; however, after Atlanta fell and Lincoln defeated McClellan in the election of 1864, all hope for a political victory for the South ended. At that point, Lincoln had succeeded in getting the support of the border states, War Democrats, emancipated slaves and Britain and France. By defeating the Democrats and McClellan, he also defeated the Copperheads and their peace platform. Lincoln had also found military leaders like Grant and Sherman who would press the Union's numerical advantage in battle over the Confederate Armies. Generals who did not shy from bloodshed won the war, and from the end of 1864 onward there was no hope for the South.

On the other hand, James McPherson has argued that the North’s advantage in population and resources made Northern victory likely, but not inevitable. Confederates did not need to invade and hold enemy territory in order to win, but only needed to fight a defensive war to convince the North that the cost of winning was too high. The North needed to conquer and hold vast stretches of enemy territory and defeat Confederate armies in order to win.

Also important were Lincoln's eloquence in rationalizing the national purpose and his skill in keeping the border states committed to the Union cause. Although Lincoln's approach to emancipation was slow, the Emancipation Proclamation was an effective use of the President's war powers.