Well, there's always "turn the other cheek." I think the consensus of most Christians is that "turn the other cheek" is not binding in all situations.
If you're talking about self-defense in a military context, the issues are more complex. In the writings of the early Christians, for the first 300 years, military service was uniformly condemned:
“We who formerly murdered one another now refrain from making war even upon our enemies.” Justin Martyr
“I decline military command.” Tatian
“We are not to draw an outline of … a sword or a bow, since we follow peace.” Clement of Alexandria
“The Christian does no harm even to his enemy.” Tertullian
“Is it lawful to make an occupation of the sword when the Lord proclaims that he who uses the sword will perish by the sword? Will the son of peace take part in the battle when it does not become him even to sue at law?” Tertullian
“So the more anyone excels in godliness, the more effective the help is that he renders to kings. This is a greater help than what is given by soldiers who go forth to fight and kill as many of the enemy as they can.” Origen
“Our prayers defeat all demons who stir up war. . . Accordingly, in this way, we are much more helpful to the kings than those who go into the field to fight for them.” Origen
“And murder – which is admitted to be a crime in the case of an individual – is called a virtue when it is committed wholesale. Impunity is claimed for the wicked deeds, not because they are guiltless– but because the cruelty is perpetrated on a grand scale!” Cyprian
“Why would [the just man] carry on war and mix himself with the passions of others when his mind is engaged in perpetual peace with men?” Lactantius
“Is the [military] laurel of triumph made of leaves, or of corpses? Is it adorned with ribbons, or with tombs? Is it wet with ointments, or with the tears of wives and mothers? It may be made of some [dead] Christians too. For Christ is also believed among the barbarians. Tertullian
After the time of Constantine the idea of military service becomes more acceptable. Today, among American evangelicals and fundamentalists, support for war is virtually universal. In fact, evangelicals support the war in Iraq by a larger percentage than that of the population as a whole.
But support for war is not universal among Christians. For example, the web site of the Orthodox Church in America notes that "when violence must be used as a lesser evil to prevent greater evils, it can never be blessed as such, it must always be repented of, and it must never be identified with perfect Christian morality."