The jihad was a sacred mission enjoined by scripture and incorporated into the holy law, to continue until all the world was open to the light of Islam. The crusade was a human enterprise, not enjoined--some might rather say forbidden--by Christian scripture, and undertaken for a limited purpose, to defend, or, where lost, to recover Christian territories.
Bernard Lewis, 'Europe and Islam' in From Babel to Dragomans, 2004.
