Generally speaking, the North's superior industrialization, I'd say that had the greatest impact on the results of the war.
Explanation:
Obviously, the north won the war, and while the southerners arguable were fighting harder, the superior industry or the north allowed them to have greater weapons, other supplies, and during that era the majority of factories and railroads existed in the northern part of the US, meaning that the north had easy access to many metals and fuels. Not only was the north at an advantage with this, but the south was at a significant disadvantage because of this as well and eventually things like trade with allies in Britain just wasn't enough to sustain the war effort.
Weak rulers bribed the army for support while Constantine led the army. Many rulers persecuted Christians while Constantine ended persecution. Rulers before Diocletian tried to rule an empire that was too large; Constantine first ruled only the West.
