This was a strong point in Foote's trilogy. I did read it, all. It's the best read of my life.
The Union won, but was never really threatened, militarily. The Union was expanding to the West at the same time, building a railroad, fighting Indians and getting into early industrialization. It was mobilized, but not like the South. The South was nothing but involved into the war. The entire CSA was devoted to a single cause, the Union otoh was a fully functioning nation.
And make no mistake, the fine gentlemen and ladies of the status quo ante bellum were the some of the most selfish, greedy and arrogant human beings since the fall of Rome. Using human bondage to enjoy a privileged life. Approx 32% of whites owned slaves in the slave states in 1860. Mississsipi and South Carolina it was nearly 50%. Confederates opposed states’ rights — that is, the right of Northern states not to support slavery.