The Civil War had its origins way back in the beginning of the 1800s. The anti-slavery movement had been steadily growing, and the economic and political split between the northern industrialists and the southern plantation owners had also been growing. Instead of working together to try to solve anything, both sides mostly just tried to force their ideals down their opponent’s throats, much as we do today (165 years later we’ve still failed to learn the lesson).
There were some compromises of course, otherwise we probably would have gone to war in the 1840s. But by the time 1860 came rolling around, there was so much built-up hatred on both sides that war was all but inevitable.
Keep in mind that the South was fighting for its very way of life. If the new territories became free states, the balance in Washington would shift and the free states would have more votes than the slave states. Slavery was on its way out world-wide, and it would just be a matter of time before slavery was abolished in the U.S. This would completely destroy the Southern economic structure as well as their social structure, both of which relied on unpaid slave labor and a working class that effectively had no rights and no voice in their own government. There was no way that the South was going to voluntarily dismantle their own social and economic way of life.
And thinking that common soldiers would say things like “we don’t follow illegal orders” is completely unrealistic. Modern soldiers might say something like that, but soldiers back in those days had a lot more respect for authority and command. Rebelling against a commander doing something extremely bad or illegal did happen back then, but it took something much more extreme to trigger that sort of behavior.
The South wasn’t just having a friendly argument with the North. They were in open rebellion. There was absolutely no question about the legality of orders in that case.
Besides, it’s not like the South could last for any amount of time. They didn’t have the heart to fight. A couple of battles and they would all go running. The war was certain to be over in a matter of days, maybe a few weeks at the most. It certainly wasn’t going to drag on for years and result in hundreds of thousands of deaths.
Uh… oops.
Yeah, both sides rather dramatically underestimated their opponent’s willingness and ability to fight. So that happened.
Yeah, this was the main problem that Lincoln faced, and it wasn’t just McClellan, though (rightfully so, IMHO) McClellan gets the lion’s share of the blame these days.
I don’t know if I would describe McClellan as defeatist, but he was definitely over-cautious to the point of losing battles that he could have won. Rather than being defeatist, I think it was more that he just didn’t like Lincoln and disagreed with him over just about everything, especially when Lincoln wanted him to be more aggressive.
Lincoln probably should have replaced him earlier, but McClellan was very popular among his troops, which made it more difficult for Lincoln to remove him.