The Time Mercenaries

the long version

There had been one war scare too many and so the human race had used genetic sorcery to delete the aggressive tendencies from its heredity. But now mankind was faced with an alien enemy so superior, so ruthless, that it was flight or be wiped out...and the humans could not fight. They couldn't even give orders for their robots to produce weapons. The only possibility was to call up and bring back to life a museum exhibit, the submarine Euphrates and its battle-trained crew. The ship had been sunk a thousand years before and had been preserved to show the decadence of violence — violence which was the only hope against an enemy to whom the living space was all-important and human life was entirely superfluous.