“Being mean makes you stupid. That's why I hate fights. You never do your best work in a fight, because fights are not sufficiently general. Winning is always a function of the situation and the people involved. You don't win fights by thinking of big ideas but by thinking of tricks that work in one particular case. And yet fighting is just as much work as thinking about real problems.”

Source: Abaddon's Gate (2013), Chapter 39 (p. 404)

Thatcher, Margaret (2002). Statecraft: Strategies for a Changing World. New York: HarperCollins. ISBN 0-06-095912-6.

Depa Billipa and Mace Windu, p. 313
Shatterpoint (2004)

Source: Wages of Rebellion: The Moral Imperative of Revolt

A Poet's Advice (1958)
Context: Almost anybody can learn to think or believe or know, but not a single human being can be taught to feel …
the moment you feel, you're nobody-but-yourself.
To be nobody-but-yourself — in a world which is doing its best, night and day, to make you everybody else — means to fight the hardest battle which any human being can fight; and never stop fighting.

Remarks to journalists in Downing Street (21 November 1990) http://www.margaretthatcher.org/speeches/displaydocument.asp?docid=108252, following the inconclusive first ballot in the Conservative leadership election.
Third term as Prime Minister

During Speech at Passing Out Parade of Indian Military Academy A Soldier's Duty http://worldofdefense.blogspot.in/2011/05/soldiers-duty-field-marshal-sam.html

This is cited to to Rommel‎'s Infanterie Greift An [Infantry Attacks] (1937) in World War II : The Definitive Visual History (2009) by Richard Holmes, p. 128, and Timelines of History (2011) by DK Publishing, p. 392, but to George S. Patton, in Patton's Principles : A Handbook for Managers Who Mean It! (1982) by Porter B. Williamson as well as Leadership (1990) by William Safire and Leonard Safir, p. 47
Disputed
Source: Rommel: In His Own Words

This is cited to Patton in Patton's Principles : A Handbook for Managers Who Mean It! (1982) by Porter B. Williamson as well as Leadership (1990) by William Safire and Leonard Safir, p. 47, but is also cited to Erwin Rommel‎ from his Infanterie Greift An [Infantry Attacks] (1937) in World War II : The Definitive Visual History (2009) by Richard Holmes, p. 128, and Timelines of History (2011) by DK Publishing, p. 392
Disputed