The Cold War and the perpetual, the ever-present notion of fear of MAD occuring, nuclear, chemical and biological annihilation of every human and animal species on the planet that most human beings lived under for nearly 45 years after the end of WWII. Two major superpowers with stockpiles of nuclear weapons enough to destroy the world 100x times over and post-apocalytpic world would resemble a charred, unlivable landscape works then than the setting than the Last of Us with Joel and (forgot her name?) criss-crossed a zombie-infected North America.