Idiom/Phrase to fight someone else’s war

The concept behind the phrase is that some areas of life are so important and overwhelming that you cannot blame someone for acting in their own best interest. For war, this implies that spies, torture, lying, backstabbing, making deals with enemies, selling out allies, bombing civilians, wounding instead of killing, and so on are "fair game" in the sense that by taking these options off of the table you are only hurting yourself: Your opponent has no reason to comply to your moral standards. (This entire concept is mostly void with regards to the current political atmosphere of Earth. Countries have actually declared certain things taboo with regards to war — with mixed success.)

The point of adding love to the list is likely to compare it to war. There are two main subtexts here. The first and most relevant is the idea that you can wreak all the havoc you want during the pursuit of true love. This includes sabotaging the third side in a love triangle or using deceit and trickery to woo the object of your affection (including hiding past lovers from them).

The second is the viewpoint that ongoing love between two people is akin to a battle that results in a dominant winner. The stereotypical gender wars are similar to this. A man and woman are in love, but a certain unease comes with the territory, and pulling one over on your spouse is fair game because, in the end, all is fair in love and war. For what it is worth, this last point is probably more of a causality (i.e., an unintended, natural consequence) of the individual words in the phrase than any original intended meaning. Most people probably don't immediately think of this type of behavior as matching up with the phrase until after they need an excuse for their actions.

Interestingly, English has almost the same expression: "fishing in troubled waters", going back to at least the 16th century.

It seems to originate in a belief among fishermen that fish rise to the surface in foul weather and may be more readily netted—hence the phrase "mackerel gale", a storm which promises a good catch.

But very early it was applied figuratively to just such situations as yours. Grafton's Chronicle and Foxe's Actes & Monuments (both 1569) quote a letter from Pope Innocent III warning King John of England not to trust those who advise him to dispute the authority of the Church:

Settle not yourself to obey their persuasions, which always desire your unquietness, whereby they may fish the better in the water when it is troubled.

I have not yet found their source for this; if authentic, it takes the phrase back to 1208 and, presumably, in Latin.