The South started the secession era as the much smaller opponent.
The strong secessionist candidate, John C. Breckinridge, obtained about 18% of the popular vote. These votes then were reflected in the undemocratic secessionist rush, and Delaware, Maryland, Kentucky and Missouri did not secede. The situation became even worse when George McClellan was able to peel off the western counties of Virginia, with the aid of the local population there.
That meant that from east to west, the chemical plants of Delaware, and the populations of Baltimore, Washington, D.C., Wheeling, Louisville and St. Louis all remained part of the federal economy and federal logistical support system.
It was not clear to the people involved, but the population mismatch was enormous.
The free institutions of the U.S. were able to hold the Democrats to the effort. The U.S. had both Congressional and Presidential elections, which were seriously contested, during the war.
It turned out that allowing dissent created strength.
This population split led directly to the US Navy, based mainly out of New York and Boston, which had no opposition along the coast, which led directly to the capture of the South's largest city, New Orleans, which had been a Douglas Democrat district.
The retention of St. Louis by the US led directly to the construction of the US river navy, which supported the effort at Fts. Henry and Donelson, and led to sudden, and irreversible loss of Memphis and Nashville.
If one stops focusing on the land battles in Virginia, and looks at the politics of the border states, and the war in the west, the realization that the advantages of the U.S. were immediately decisive becomes clear. Land battles simply don't matter if they don't create a strategic advantage. All the strategic advantages were flowing to the US as soon as the secessionist rush subsided.
Although the war was prolonged by US hesistancy and error in 1862, the war 1863 illustrated the problem.
General Grant was operating deep in Confederate territory with tremendous naval and logistical support.
After numerous experiments he finally hit on a tactical combination that worked, as he was bound to do.
In Pennsylvania, the AofV found out that the US army was just too big, and too experienced to be conquered.
The mismatch in manpower mattered as soon as the AoftheP stopped making mistakes.
The situation in Tennessee exposed all the problems with which the Confederates had no solution.
The US used its railroad system to place and support four armies, the Ohio group in Knoxville, the AofTenn under Sherman, the Cumberlanders under Thomas, and Hooker's eastern divisions, in a confined area in Tennessee. Not only was the US' railroad system able to support these armies, but local people in eastern Tennessee were willing to help Burnside's army in Knoxville.
Leadership and tactics make some difference, but the situation around Chattanooga in November 1863 was a direct reflection of the mismatch in logistical strength of the adversaries.
These things were a direct reflection of the geographical problems faced by the South.
In the west, in the inland river system provided a ready pathway of invasion for the US. Stationary forts to block use of this route were readily overcome by the US.
In the center of the Confederacy, the Appalachian mountains isolated the more heavily populated Atlantic coast from the larger territory of the west. The Confederacy operated as two separate areas, and the eastern area always obtained the best of the resources. The US exploited this division consistently.
The eastern press, based in New York, sold the idea that it was a close contest. The Confederate leadership wanted to maintain the idea that the attempt at rebellion had a substantial chance of success.
However it was never a close contest.
All of this without mentioning that about 40% of the population in the South was a built in resistance movement.
The black enslaved population steadily shifted away from the South and towards the US and it made a material difference in hastening the end of the war.