Last Stand — Keith Laumer

LIKE A ROCK
(But then, the first Bolos were GM products....)

Controlled by their tireless electronic brains which were programmed to admit no possibility of defeat, the gigantic robot tanks known as Bolos were almost indestructible, and nearly unstoppable. Almost. Nearly. A sufficiently determined enemy armed with nearly limitless firepower and willing to sustain terrible losses could destroy a Bolo. But even a terminally damaged Bolo is still an opponent to reckon with. And as long as a Bolo's artificial intelligence retains a flicker of consciousness, its indomitable drive to defend the human race against all enemies will propel it forward. Bolos can be destroyed--but they never surrender!