Laumer Keith. The Compleat Bolo

Even in the future, the classic mercenary dilemma remains: can you trust your servants not to turn on you? The servants protecting mankind and the Terran Empire are the Bolos - gigantic battle tanks with robot brains. When the alien Deng expand into human space, the Bolos counter-attack. But they seem to have a war plan all their own, one that doesn't take humanity into account.