Definition
This idiom suggests that in the contexts of romantic relationships and warfare, normal ethical or moral rules of conduct do not apply, and any actions are permissible to achieve one's goals.
They were using every possible tactic to win the client, but as they say, all's fair in love and war.
He deceived his friends while pursuing her, but he figured all's fair in love and war.
When it came to getting promoted at work, she played very hard, because she believed all's fair in love and war.