The proverbial saying ‘All’s fair in love and war’ expresses the idea that, like war, where any strategy is accepted, affairs of the heart are also no-holds-barred contests.

In common with many proverbs, the basic idea behind ‘All’s fair in love and war’ existed in other forms before it took on the wording we now use. These variants are often in other languages which may supersede the English version.

The first known example of the proverb I know of is in the English writer and courtier John Lyly’s romantic novel Euphues, 1578:

Anye impietie may lawfully be committed in love, which is lawlesse.

Soon after the end of the Tudor era the Irish translator Thomas Shelton published an English version of Cervantes’ Don Quixote, 1620:

Love and warre are all one [Spanish – el amor y la guerra son vna misma cosa]: and as in warre it is lawful to use sleights and stratagems to overcome the enemy: So in amorous strifes and competencies, Impostures and juggling tricks are held for good, to attaine to the wished end.
Almost a century later we find a version of the proverb that is close to its present form – in The Artful Husband, 1717, the comic play by the English lawyer and playwright William Taverner:

All advantages are fair in Love and War.