If there is a principle for Christian pacifism, it is this: you are already dead.
Too often we read passages in the Scriptures, or sing songs, or pray prayers that assert we have died with Christ and our life is hid in Him, but we take this as metaphorical, which is to say, as a pious fiction meant to be psychological balm. So we feel good that after our biological cessation, we will be in heaven. Maybe. More often, I've noticed, we fear death in the same measure as pagans, some times we fear it more. But the Scriptures are clear, the Tradition is clear, the lives of saints from the Apostle Peter to our new Iraqi and Syrian martyrs are clear, this is no mere metaphor: it is the truest possible truth there is. We have already died.
Why are we so violent? What do the protestors in Ferguson, or the police, fear so much that they would abjectly deny the humanity of the other? Death. I do not mean to be too simplistic, although that is how I will be read by some. We know, in the core of our being, that death is profoundly unnatural (even though, in one sense, it is the most natural thing): so we do whatever we can to forestall it, even at the expense of another human's life. The fear of death combines with what the Fathers call philautia, or love of self, to exclude (that is, kill) the whole world for the sake of a few moments more for us, or for our tribe, or for our nation. But those things, proleptically in Adam and truly in Christ, are dead too. This is why Christ can call us to "leave house or wife or parents or brothers or children for the sake of the Kingdom of God" (Lk. 18:29): these allegiances, all of which dissolve upon death, are already dead (and reborn, as He goes on to say). But this leaving isn't some sort of Pilgrim's Progress abandonment, but rather a death of self, a taking up of the Cross, the place where Christ called even for forgiveness towards those who crucified Him (and Christian history with the Jews would well learn to remember that the Father always grants the prayers of the Son), the only safe place in the entire universe, as the greatest power man, whether government official or rioter, possesses has no power there. Christ has trampled down death, the fell weapon, by death, and upon those in the tombs bestowed life that they might lay it down for the Kingdom of God.
This leads to the active principle of Christian pacifism: interposition.
The pacifist, who understands and knows himself to be dead in Christ and his life hid thereof, can then act as a witness to Christ's resurrection by placing his body between warring parties. Just as Christ interposed Himself between us and death, so we can can place ourselves in His stead (as His Body) between those who kill out of the bondage of the fear of death. While this language might not be used, this is the implicit faith of the martyrs, past and present. They interposed -- an act of intercession before God -- themselves between the demons who have always sought to influence those in power and the corrupted image bearers wielding power. St Ignatius of Antioch comes to mind here as a specific example of one whose faith and witness to Christ's resurrection remade the violent and depraved Roman Empire.
Martyrdom, in other words, is the base of Christian pacifism. This is no wishy-washy, bleeding heart liberalism; this is the faith of Christ and His Church, shown to us by the martyrs for the sake of the world.