How did the the union's goals in the civil war change between 1861 and 1865? what accounts for those changes?

At first Lincoln's goal was just to bring the southern states back into the Union as quickly and easily as possible. But as the war continued on he realized that the South was not going to come back into the Union that easily. Many of the Union's generals underestimated the south as well. That was when Lincoln's goal moved to destroying the South's ability and willingness to fight. Grant was one of the few who realized this and tried his best to break the south down. It is said that when Sherman (a major general in the union) realized what it was going to take to bring the Civil War to a successful conclusion, he had a major breakdown from the enormity of the problem.