Will & Ariel Durant

Power dements even more than it corrupts, lowering the guard of foresight and raising the haste of action.

Will & Ariel Durant, The Story of Civilization: The Age of Napoleon (1975)‎, Ch. IV: The Convention: September 21, 1792 — October 26, 1795, Part V: The Reign of Terror: September 17, 1793 — July 28, 1794, § 4: The Revolution Eats Its Children.