Turn the other cheek is the most used and abused expressions in the Bible.
The Greek used implies a blow with the back of the hand, ie an insult, more than a physical attack. In other words, don't let pride be the motivation for striking back.
An enemy who kills 5000 people is another matter all together.
That said, we get them. As they deserve.
A more accurate way of putting it is that 'turn the other cheek' never was intended to be used in the way it has been corrupted, ie the 'Socialist Abridged Version of the Bible'.
It really is not an admonition to let somebody knock you around, but more of an admonition to not let people insult your pride, and then proceed to act from this motivation.