Definition
This idiom suggests that in the contexts of romantic pursuits and warfare, normal ethical standards may be disregarded.
He resorted to cheating to win the competition, after all, all's fair in love and war.
When the competition got fierce, they seemed to forget about ethics, as they thought, all's fair in love and war.
She used various tricks to pursue the person she liked, because, as the saying goes, all's fair in love and war.