Why did the Union army suffer so badly on the offensive compared to the Confederacy? I'm going through Shelby Foote's Civil War narrative and a practically recurring theme is the Army of the Potomac getting a promising new general, marching south to smash the Confederates and take Richmond, and receiving a solid beating before trudging back north to recover and await yet another general. McClellan, Pope, Burnside, Hooker, and Grant have all so far marched south the army to be badly bled without much gain except for Grant (I just finished Cold Harbour), and with the latter especially taking brutal losses in proportion to Lee. 

While the CSA I know never made a successful offensive north, Antietam and Gettysburg don't seem to have been as decisive of defeats as Chancellorsville or Fredricksburg, and overall the CSA leadership seems to be much better of the defensive than Union. Why does this seem to be the case? 