Bradbury indicates in Fahrenheit 451 that the futuristic American society he describes is predatory on the rest of the world and has repeatedly attacked other countries to dominate them and maintain its standard of living. This, in fact, is only an outward reflection of the way the society behaves internally -- hedonistic, selfish, callous, and uncaring of damage done to others. The ending of the book strongly suggests that the final destruction of the city by a nuclear bomber attack is the result of these endless wars, one of the enemy nations finally being able to strike back and take revenge.
I think that Bradbury used war as a way to convey that the whole world was in chaos. To fully illustrate that society was devoid of order. It adds to the other symbols of a world gone mad. Such as fireman who start fires. Mechanical dogs that hunt down people and kill them. Books are illegal and must be burned.
War is a perfect setting for the upside down violent, cruel jpolice state that exists in Fahrenheit 451.
Ray Bradbury was totally against any form of censorship. Consider that the book main theme is that the written word is forbidden, as a writer, Ray Bradbury would consider that a declaration of war.