Why did the south fight a defensive war?

The dominant military theories of the time said that the defensive position was usually superior because the defensive side could concentrate itself in "interior lines." This was the classic rule of the French strategist Jomini, whose teachings dominated West Point instruction before 1860.To picture what "interior lines" mean, imagine your army fighting inside a circle that is one-mile around. You will never have to travel more than about one-third of a mile to go from one side of the field to the other, while your opponent has to run a half-mile on the outside to get from one side to the other. Therefore a smaller force within interior lines has the advantage and is likely to defeat the aggressors.

Jomini aside, the South believed a defensive war was wise because of its vast territory. The invading Federals would be swallowed up and lost like Napoleon in Russia.

Finally, there were diplomatic and political considerations. Seeking recognition from France and Britain, the Confederacy believed it held the moral high ground if it refused to attack the Northern side. Unfortunately this meant it missed the opportunity to win the war, as it had several opportunities to capture Washington, Baltimore and maybe Philadelphia in its first year, but afterwards the Federal side was too well equipped and trained.