There are two possible approaches here:

1. Selective Pacifism understood as a willingness to fight only when one is convinced that a cause is just (and not just when you receive the call up papers from a legitimate authority). This seems to have been Martin Luther King’s position as outlined in a famous sermon “Against the Vietnam War” which can be listened to on JustWar.com.
2. Thoroughgoing Pacifism understood as a an unwillingness to fight anywhere, at any time, on any issue. This was the approach taken by Bertrand Russell, for example, and caused him to be placed in solitary confinement for a year in 1917. Even he wavered, however, when faced with the evils of Hitler.

Stanley Hauerwas is an American theologian who has written extensively on ethics. In his essay in the journal “Faith and Philosophy” (April 1985) he lays down the philosophical basis for pacifism. Hauerwas argues from a particular standpoint, that of Jesus’ view of the kingdom of God, a kingdom in which the rule is not through violence but through love. He argues that this kingdom revolutionizes our ideas of human relationships, so that we are obliged to love both the weak and strong, the attacker and the attacked. “God calls us to be part of his rule by calling us to a community that is governed by peace”, and we must focus on God’s way as shown by the cross, “just as God refused to use violence to ensure the success of his cause, so should we”.

Hauerwas stresses that pacifism is not the same as passivity. To the charge that a pacifist acquiesces in injustice and violence by standing by as an observer, he argues that the Christian has no side except the side of God and his rule.

This rule, which is appropriated by faith involves such radical ideas as turning the other cheek, loving your enemies, and going an extra mile (See Matthew 5). The argument of the New Testament could be summed up thus: by following the example of Christ and his cross we eventually convert even our enemies, even at the risk of our own deaths.

Such radicalism would indeed take moral courage.

For an excellent summary of religious attitudes to war go to:

http://www.ppu.org.uk/learn/infodocs/st_religions.html