At the outset of the Civil War, the Confederacy had several advantages. Firstly, the Confederate Army was initially composed of experienced officers who had previously served in the United States Army, and the Confederacy also had a higher proportion of military-aged men than the North. Additionally, the Confederacy's economy was heavily dependent on agriculture, which meant that it was more self-sufficient in terms of food production and less vulnerable to blockade than the industrialized North. Moreover, the Confederacy initially had better access to firearms and ammunition, as well as better-trained cavalry units.
The Union, on the other hand, had several advantages entering the war. The North had a more significant population, which meant that it could field larger armies and had more resources for the war effort. Additionally, the Union had a more robust industrial economy that could produce weapons, ammunition, and other necessary supplies in greater quantities than the South. Furthermore, the Union had a more extensive network of railroads, which allowed for more efficient transportation of troops and supplies.
Two key causes, events, and/or decisions that allowed the North to gain the upper hand and win the war were the Emancipation Proclamation and the Battle of Gettysburg. The Emancipation Proclamation, issued by President Lincoln in 1863, declared that all slaves in Confederate-held territory were to be freed. This move fundamentally changed the nature of the war and gave the Union a moral cause to fight for, which helped galvanize support for the war effort. The Battle of Gettysburg, fought in July 1863, was a significant turning point in the war. The Union's victory in this battle marked the end of the Confederacy's efforts to invade the North and forced it to fight defensively for the rest of the war.