Still, if you will not fight for the right when you can easily win without bloodshed; if you will not fight when your victory will be sure and not too costly; you may come to the moment when you will have to fight with all the odds against you and only a precarious chance of survival. There may even be a worse case. You may have to fight when there is no hope of victory, because it is better to perish than to live as slaves.
Winston Churchill, The Second World War, Volume I : The Gathering Storm (1948) Chapter 19 (Prague, Albania, and the Polish Guarantee)
And because he’s so damned quotable:
Never give in — never, never, never, never, in nothing great or small, large or petty, never give in except to convictions of honour and good sense. Never yield to force; never yield to the apparently overwhelming might of the enemy.
Ibid, speech given at Harrow School, Harrow, England, October 29, 1941.