H.G.Wells' The War of the Worlds
A nightmarish but surprisingly accurate vision of what mechanised warfare could become. He predicted the use of nerve gas, lasers (or flamethrowers, depending on interpretation), laser targeting, 'silent' communication (ie scrambled radio), modular weapons systems, combat aircraft and even dropships, something that still doesn't quite exist today.
He also predicted the one sided nature of modern warfare, where the inferior side striking back usually just means more potent weapons brought against them.
That it's a gripping story in of itself doesn't hurt either.