My best friend in high school back when this and 'battle" were on CBS the first time speculated about how the "between movies" war started. He suggested that, during the uprising, some revenge crazed ape inadvertently hit a launch button.
I don't even know enough to know how much I don't know. Ya know?
Actually it was started, or dreamed up to be more accurate, by the folks at Twentieth Century Fox who suddenly realized a bunch of apes raising hell would not explain how the world came be like it was in "Planet of the Apes" and "Beneath the Planet of the Apes". They also wanted to make one more apes movie so they could pay off the cost of all those monkey suites. Having the apes and humans doing a whole movie about shacking hands and singing around a campfire would be a bit boring.
Seriously sort of the fact there was a war was established at the end of Planet of the Apes. Maybe a crazed ape, or the bomb worshiping dudes from "Beneath the Planet of the Apes, got the launch codes. Or maybe, as normally happens in Hollywood, when the world gets nuked, its a self aware computer, a dude who thinks fluoride in the drinking water is massing up his sex life or some one spills Coke glass over a control panel.
TAG LINE: True genius is a beautiful thing, but ignorance is ugly to the bone.