ALL IS FAIR IN LOVE AND WAR

Origin
This phrase, in its current form, was first found in the novel “Frank Fairlegh” by Frank E. Smedley in 1850. Very similar phrases with the same meaning are found in the 1620 translation of Miguel de Cervantes’ Don Quixote II by Tom Sheldon and the 1845 novel Smuggler II by G.P.R. James. Even prior to that, love has been equated to lawlessness and trickery since 1578-1579. Source: theidioms.com