All's fair in love and war

Cf. [1578 Lyly Euphues I. 236] Anye impietie may lawfully be committed in loue, which is lawlesse.

Love and warre are all one. ‥It is lawfull to use sleights and stratagems to‥attaine the wished end.
[1620 T. Shelton tr. Cervantes' Don Quixote ii. xxi.]

In love and war, every stratagem is fair, they say.
[1845 G. P. R. James Smuggler II. iv.]

Then Eulalie's eyes narrowed and she looked hard at her companion. ‘Do you believe that all is fair in love and war?’
[1986 S. Brett Nice Class of Corpse xl.]

All is fair in love and war, and it is important that you sustain your marriage. You must therefore take the following, deceitful steps.
[2002 Spectator 15 June 63]

Related to: fair dealing; love; warfare

Bibliography of major proverb collections and works cited from modern editions is available here.

Any conduct is permissible in certain circumstances, as in Of course he called her--all's fair in love and war. This maxim, stated in various forms from 1579 on, today sometimes appears altered by an addition or substitution, as in All's fair in love and the World Series, or All's fair in love and war and an election year.