During the civil war, this region had the advantage of being home to two-thirds of the country's population, giving it more available men for military service. a) north b) south

This is an excellent question to study in some detail
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You want to know that population mattered in every war up to and including the second world war. Stalin ultimately defeated Hitler because he used women in the military and his population base was much larger to draw people from. Russia lost as many as 25 million people by some estimates. Hitler though in the beginning had better trained more experienced troops, but that eventually didn't matter. Russia just wore Germany down by putting more people into battle to replace the ones they lost.

The exact same thing happened between north and south in the American Civil war. I happen to know a bit more about WWII than I do about the civil war, that's why my example is from there.