The League Parliament was the governing body of the League of Nobles. This body was effective in handling the defense of the League before the Butlerian Jihad. However, it's large bureaucracy was not conducive to swift and decisive action in the events leading up to and the conduct of the Butlerian Jihad.
As a result, the Jihad Council was formed to provide an organ with the ability to make swift decisions and act upon them with out the endless hearings and committees that bogged down the League Parliament. Iblis Ginjo intended for the Jihad Council to absorb more and more power from the Parliament and ultimately replace it.