all of the above
Vicksburg was the key to the Anaconda Plan's goal of gaining control of the Mississippi River and cutting the South in half.
So true, although the Winfield Scott plan was laughed at when he suggested it, it had to be clear that controlling the Mississippi River was vital to the North's war efforts.
Iron clad ships were introduced into battle
New technology applied in the war.
Food shortages
An effect of the Civil War was that slavery had effectively ended.
Sherman