taken from 'the art of war', by sun tzu, which is aN ancient book from the east about the spiritual side of war.part III: Attack by StratEgy says at line 18:

Hence the saying: If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.

'know your enemy' is also an anti-american song by rage against the machine on their first album.