“Mortal combat corrupts, and war corrupts absolutely.”

Slightly edited version of text in Men and Women of the Corporation, Basic Books, 1977

Letter http://oll.libertyfund.org/index.php?option=com_content&task=view&id=1407&Itemid=283 to Mandell Creighton (5 April 1887), published in Historical Essays and Studies, by John Emerich Edward Dalberg-Acton (1907), edited by John Neville Figgis and Reginald Vere Laurence, Appendix, p. 504; also in Essays on Freedom and Power (1972)
Paraphrased variant: All power tends to corrupt and absolute power corrupts absolutely.
Context: I cannot accept your canon that we are to judge Pope and King unlike other men, with a favorable presumption that they did no wrong. If there is any presumption it is the other way against holders of power, increasing as the power increases. Historic responsibility has to make up for the want of legal responsibility. Power tends to corrupt and absolute power corrupts absolutely. Great men are almost always bad men, even when they exercise influence and not authority: still more when you superadd the tendency or the certainty of corruption by authority. There is no worse heresy than that the office sanctifies the holder of it. That is the point at which the negation of Catholicism and the negation of Liberalism meet and keep high festival, and the end learns to justify the means.

Source: Lullaby (2002), Chapter 9

Cold Colors (p. 239)
Smoke and Mirrors (1998)

The Guardian interview (2002)

This quote is often misattributed to John Lehman, but it apparently was actually said by Donald Regan, President Reagan's chief of staff, to a 1987 Gridiron Dinner audience. http://www.johnflehman.com/books/books_makingwar_vulnerable.html
Misattributed

Address to Transparency International Fiji book launch, 9 December 2005.