north and south disadvantagis
To stand on the defensive meant that the North had to invade and carry the fight to the South. This allowed the South to prepare and receive the enemy, although Lee was not a passive man by any standards.
The South did not have many advantages, but just like in the American Revolution, it's very difficult to conquer an area. Essentially the South only had to muster the will to keep fighting until the North felt like quitting. That didn't happen, but theoretically it could have.(But it did not, for multiple reasons. Including the lack of previsions, lack in number of men, & a superior navy & rail road system.)
When Southerners said that they merely wanted to secede from the Union they gave themselves a military advantage. Thus the North had to attack.(The North did not attack first, the South did.)
The geography of the South favored the defender over the attacker. In particular, most of the rivers in the South run east to west. Thus as Union forces attempted to move south they had to cross all the rivers and the Confederates could defend them. Also, the fact that the Cenfederacy was simply defending itself meant that the Northern forces had to invade and occupy virtually the entire South before they could claim victory. That was a monumental task.(The North had a 3 part plan to defeat the South. 1) blockade the south 2) split the south using the MR river 3) take control of the capital of the confederates. Geographically speaking, the South was exposed.