Well, in that case I would say that the behind-the-scenes diplomatic battle between the North, South, England and France was more important than any battle fought with weaponry in the Civil War. Secretary of State Seward went to Europe and made it very clear that if either of those European powers were to enter the war, the North may well lose to them, but the North would also take as many Europeans with them as possible. This threat kept England and France out of it until it essentially became too late for them to enter and have any real impact without inevitably severe damage to themselves. By mid-1862, the risk to England and France had become much greater than the reward.
At the same time the South decided to try to lure these powers into war by raising their cotton prices. The South mistakenly thought that this huge spike in price would give England and France a reason to enter on the South's behalf, if for no other reason than to lower the price of cotton back down for them. The South knew that these powers did not want to see our little experiment in democract succeed and they thought the cotton prices would be the thing to tip them over the edge and pull them in. But this plan backfired on the South when England and France said fuck it and bought their cotton from Egypt instead.
If either England or France had entered the war on the side of the South, the North would have been engaged in a war with four fronts (north, south, east and west instead of primarily to their south, and in the west to a lesser extent). But Seward was able to keep them out of the war. If he had failed in this diplomatic battle and the South had won it, there would never have been a Gettysburg, there probably wouldn't be nearly the same form of democracy that we have today, England and France would probably be the dominant superpowers today, etc, etc.
The most important battles in modern history aren't fought with guns and bullets.