What did the American Civil War last as long as it did? Shouldn’t the North’s superiority in industrial capability and manpower have made it a lot shorter than it was?
First the North had to overcome its tendency to pick less than optimal generals, which led to a bit of a bad start.
Then, the North had to subdue an agrarian economy populated by people who were simply not interested in giving up–requiring an actual invasion, not merely a destruction of industry by raids, something that requires a lot of time and effort. WWII, which had only a slightly shorter duration had turned the corner and put the Japanese on the defensive in fewer than twelve months. The Gettysburg/Vicksburg turning of the tide in the Civil War did not occur until more than two years into the struggle.
The South was on the defensive; the North had to actually do something (invade) to defeat the secession. Also, the United States had no standing army to speak of in those days, so both sides had to build up military forces from practically nothing, and although both sides eventually wound up with some fine professional military commanders, there was a lot of amateurishness, and in particular it took some time for the North to realize what its own “superiority in industrial capability and manpower” meant–i.e., that the North could and should wage a bloody war of attrition.
Also, Americans just seemed to be a lot more bloody-minded back then–I don’t know if that was because of no TV piping battles into people’s living rooms, or high infant mortality rates and young adult death rates, or what; but the First Battle of Bull Run resulted in about 4,800 casualties (over 800 dead), total for both sides. I recall from Ken Burns’ Civil War documentary series that up until the battle, a lot of people looked on the whole thing as a glorious adventure, but Bull Run sobered everyone up a good bit. But, instead of saying “Oh my God, the horror–hundreds dead; thousands wounded! We can’t go on! We must abandon our attempt at independence/Accept the South’s independence!”, everyone on both sides settled down to four years of killing and hundreds of thousands dead or maimed, which was apparently considered acceptable to preserve the Union (and on the other side, the bloodshed was considered acceptable to establish Southern independence for a long time, until the South was clearly beaten–its armies in the field surrendered, and its heartland invaded and ravaged).