The North thought the war would be over in 3 months, IIRC.
Neither side forsaw the length and bloodiness.
Each side thought they only had to show up and the other side would be so impressed that they would run away, and the “war” would be over in six weeks. What was the real “killer” was the South really didn’t count on the North sticking with the fighting once it got tough. I think the Northerners even surprised themselves with their determination.