Even after it became clear that there would be no quick victory in the Civil War, it was not a foregone conclusion that the Union would prevail over the Confederacy.
Between 1861 and 1865, the North and the South waged war against each other in what remains the bloodiest war in American history. For much of that time, it was not entirely clear which side would win. Both sides enjoyed their own unique advantages which they believed would be enough to ensure a quick victory. Even after it became clear that there would be no quick victory in the war between the states, it was not a foregone conclusion that the Union would prevail over the Confederacy. As the war progressed, it seemed more and more inevitable that the North would prevail, but that was not the case at the beginning of the war.
Looking back, it is easy to see why the Union won. The Union population of 22 million was greater than the Confederate population of 9 million by more than two to one. That allowed them to field much larger armies and hold many more troops in reserve. Further, the North was better able to supply those armies because it produced far more than did the South. Whereas agriculture was the cornerstone of the Southern economy, commerce and industry were more important to the Northern economy. That meant that North could produce the things it needed to prosecute the war itself while the South had to rely upon trade with foreign countries to procure those necessities. Given that the North had a naval advantage over the South which allowed it to conduct a successful blockade of the Southern ports, the South was unable to procure much needed supplies. Eventually, the South had to surrender not because it had been destroyed militarily, but because its economy had been effectively destroyed by the North.
The South did have at least two important advantages in their favor that made a Union victory far from certain in 1861. Perhaps the most important was the fact that they had better military leaders. Most of the best military officers in the United States military prior to the outbreak of the Civil War were from the South. When the Southern sates succeeded, most of these officers such as General Stonewall Jackson and General Robert E. Lee resigned their commissions and accepted positions in the Confederate Army. The Union still had some good officers, but not as many as the Confederacy and poor leadership was one of the primary causes of early Union defeats.
The South also had something of a “home field” advantage. Since the North was invading the South, the Southerners had to fight close to home. Often, they were much more familiar with the terrain than were the Northerners and they were always more desperate to win. When you believe that you are defending your wife and children from invaders, you care more about your cause than those who are simply being paid a wage to go fight for a cause that does not influence their lives as directly. This made a significant difference on the battlefield.
If the South and the North had the same population and the industrial output and the beginning of the war, there is little reason to believe that the Union could have defeated the Confederacy. Outnumbered and outgunned, however, the Confederacy found their ability to make war slowly whittled away by the Union. Many things could have happened to change this slow bleed type of warfare. If, for example, several of the key Confederate generals had remained loyal to the Union, the war might have been over much more quickly and not have been nearly as destructive. As it was, however, the strengths of both sides largely canceled out each other and resulted in a draw. The North had more resources that allowed them to continue the war longer than the South could, but only after a long, bloody fight.