If you “fight fire with fire,” you respond to an attack by using similar methods to those of the attacker. In other words, match aggression with aggression. Meet violence with violence. Take an eye for an eye.
But the phrase is normally used for the controlled burns and backfires that wildlife managers use to burn fuel that could potentially feed a future wildfire. The burn creates a manmade firebreak, or gap, in combustible material to contain spreading wildfires. A fire needs oxygen and fuel, such as leaves and vegetation, to continue burning. Rob the fire of either source of nourishment and you squelch the chemical reaction that produces it.
The source of this phrase was actual fire-fighting by US settlers in the 1800s, though Indigenous Peoples have been literally fighting fire with fire since time immemorial. They tried to guard against grass and forest fires by deliberately lighting small controllable fires, called “back-fires,” to deprive a larger fire of fuel. This literal ‘fighting fire with fire’ was often successful, although such fires sometimes got out of control and made matters worse rather than better.
The earliest use of ‘fight fire with fire’ in print appears to be US author Henry Tappan’s 1852 reminiscence A Step from the New World to the Old, and Back Again: “Smoking was universal among the men; generally cigars, not fine Havanas, but made of Dutch tobacco, and to me not very agreeable. I had some Havanas with me, and so I lighted one to make an atmosphere for myself: as the trappers on the prairies fight fire with fire, so I fought tobacco with tobacco.”
Don’t worry about the burned vegetation, though. Grass and forest fires are a natural occurrence. In a world free of humans, they’d still occur thanks to lighting strikes, sparks from falling rocks, volcanic activity and the spontaneous combustion of organic materials. While an occasional burn might greatly inconvenience local human populations, it’s all a part of the natural ecological cycle.
Some plant species actually depend on fire as part of their reproductive cycle, while others evolved long ago to weather regular wildfires. Sequoia seeds, for example, actually remain dormant until fire breaks down the seeds’ outer coating. As such, a good controlled burn can also aid the environment by stimulating local vegetation.