From: Matthew 5:38-42
Jesus and His Teaching, the Fulfillment of the Law (Continuation)
(Jesus said to His disciples,)  "You have heard that it was said, `An eye for an eye and a tooth for a tooth.'  But I say to you, Do not resist one who is evil. But if any one strikes you on the right cheek, turn to him the other also;  and if any one would sue you and take your coat, let him have your cloak as well;  and if any one forces you to go one mile, go with him two miles.  Give to him who begs from you, and do not refuse him who would borrow from you."
38-42. Among the Semites, from whom the Israelites stemmed, the law of vengeance ruled. It led to interminable strife, and countless crimes. In the early centuries of the chosen people, the law of retaliation was recognized as an ethical advance, socially and legally: no punishment could exceed the crime, and any punitive retaliation was outlawed. In this way, the honor of the clans and families was satisfied, and endless feuds avoided.
As far as New Testament morality is concerned, Jesus establishes a definitive advance: a sense of forgiveness and absence of pride play an essential role. Every legal framework for combating evil in the world, every reasonable defense of personal rights, should be based on this morality. The three last verses refer to mutual charity among the children of the Kingdom, a charity which presupposes and deeply imbues justice.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.