The North’s population of 22 million pitted against the 9 million Southerners seemed like an overwhelming majority, and then given that 3 million of the 9 mil. Southerners were slaves, the North obviously had far and away more people. Abraham Lincoln was also more capable than Jefferson Davis (arguable, but generally accepted). The Union had the “Anaconda Plan” to blockade the South and stop all exports. Finally, the North was an industrial giant, with far more factories and railroads (needed for fast troop movement) than the South.
The Confederacy possessed advantages of its own. Most of the veterans from the Mexican War were Southerners, because they were more concerned about Mexico, being closer. Also, Southerners, unlike Northerners, were mostly brought up in the countryside, and so were familiar with horses and guns. Robert E. Lee, Stonewall Jackson, and a host of such generals were Southerners. The Southern troops also possessed immense tenacity and will to fight, because they believed that they were fighting for their rights to live as they wished. Finally, even though the South lacked the railroads the North had, the South could still move troops on “interior lines” faster than the North had to move them. There was also hope that “King Cotton” would force England to break open the Union blockade to let cotton in.
As the fighting began initially, both sides had approximately equal troop numbers, but the South had to begin drafting a year earlier than the North. With this equal, and some rifles captured from the Union and smuggled from England, the Confederacy gained some strategic advantages. Their “generalship” was far superior, with Lee and Jackson easily outclassing bumblers such as Burnside and McClellan.
Later in the war, this picture changed. The South had to “scrape the barrel” to get soldiers, while the Northerners invented new repeating rifles (the first submachine guns) that easily out-shot any of the single-shot muskets the Southern soldiers carried. Grant, Sherman, and Thomas gained prestige in the eyes of Lincoln, and so could match the Confederate generals, half of which had already been killed off. The Confederacy also lost large chunks of territory, including northern Virginia, Tennessee, Georgia, and all 3 states east of the Mississippi river. At the close of the war, the North had more than 5 times as many men in the field than the South, with the two major Confederate armies reduced to shambling, tattered mobs. In the end, the “dry goods clerks”, backed by heavy industry, triumphed over the “country farmers”, with only a limited and exhausted fighting spirit.