The saying "all's fair in love and war" is a little over a century-and-a-half old, but the idea of comparing love and war is a couple centuries older still. Miguel de Cervantes made the comparison in 1604 in Don Quixote
when he wrote, "Love and war are all one . . . It is lawful to use sleights and stratagems to . . . attain the wished end." (This is, of course, translated from the original Spanish.)
The comparison of love and war appeared in literature numerous times over the next 250 years after Don Quixote became a hit. Not until 1850, though, did we see the exact phrasing that we know today. Frank Smedley wrote in his novel, Frank Fairlegh: Scenes from the Life of a Private Pupil:
"You opened the letter!" exclaimed I.
"In course I did; how was I to read it if I hadn't? All's fair in love and war, you know . . ."