I spent most of my formative years in a Mennonite church, and identify with the Mennonite concept of pacifism. So I will attempt to answer from a Mennonite Pacifist perspective, based primarily on my understanding, as taught to me by Mennonites, of this view (as opposed to my personal opinion on this view, which does vary slightly on a few points), and when possible, will provide references.
First, one must define what is meant by pacifism (and in this case, specifically, Mennonite Pacifism). First, one thing that pacifism is not.
Pacifism is not a "passive" approach to aggression
While "pacifism" bears a resemblance to the often confused concept "passivism", they are not the least bit related. The root of the word "pacifist" is the same as to "pacify" or "peace," whereas the root to the word "passivism" is "passive".
While few people go so far as to claim that "pacifism" comes from the same root as the word "passive" (although I have seen this argued before!), there is still an underlying concept in pacifist critique, that pacifists are, by and large, weak cowards who run from a fight and are, indeed, "passive" when faced with the threat of violence.
Pacifists, especially Mennonite pacifists, are anything but passive! They are generally very active, and even offensive, in their pacifist activism!
An article in The Mennonite magazine describes pacifism as:
[P]acifism focuses on living without harming others at the minimum and as much as possible loving enemies and doing good to those who harm you.
Notice that this doesn't say anything about passively taking a beating from an aggressor!
What is Pacifism then?
If Pacifism is not responding to aggression with violence, and pacifism is not being passive, what is it?
Pacifism is the pro-active non-violent response to aggression. It can take many forms. Some simple examples:
- Non-violent protests
- Smuggling of unjustly persecuted people (such as Jews during the Holocaust, or blacks in the US underground railroad) to hide them from a violent/oppressive regime
- Civil disobedience
- Speaking against acts of violence
- Countless others
Naturally, most of these activities attract a lot of negative attention, especially from those who are in favor of a violent response to an aggressor or threat.
What guides a Christian Pacifist?
A Christian Pacifist holds a few Biblical principles in very high esteem. I will spell these out, then help show how they can be applied to a 'rampant evil' situation, such as WWII.
Love your enemy.
love your enemies and pray for those who persecute you Matthew 5:44 NIV
This is probably the key verse for Christian pacifists. If we take this commandment from the Sermon on the Mount to heart, it becomes practically impossible to kill our enemy (unless we can somehow justify a mercy killing, but that's ruled out by the next point, which is... )
You shall not murder.
You shall not murder. Exodus 20:13 NIV
Turn the other cheek
You have heard that it was said, ‘Eye for eye, and tooth for tooth.‘ But I tell you, do not resist an evil person. If anyone slaps you on the right cheek, turn to them the other cheek also. Matthew 5:38-39 NIV
Here Christ tells us directly do not resist an evil person. This is surely applicable to 'rampant evil', as much as it is to a rude friend who slaps you in the face. Not only are we told not to resist, we are told to make ourselves even more vulnerable to them!
Violence is for God, not man.
Do not take revenge, my dear friends, but leave room for God’s wrath, for it is written: “It is mine to avenge; I will repay,” says the Lord. Romans 12:19 NIV
This won't always apply to war situations, but when you consider those who are most quick to criticize pacifists in times of war, it is those who are motivated by a drive for vengeance.
These verses, and others, when considered together in the context of the Bible, and especially the new testament, are taken to paint a picture of non-violent resistance to oppression and aggression. Christ himself lived under an oppressive regime, and among would-be violent revolutionaries (many whom did become violent after Christ's ascension), who even asked him to legitimize their fight against Rome, and Christ responded by saying "Give unto Caesar what is Caesar's"--hardly fighting words!
So both the commandments given by Christ, especially the one about loving one's enemy, and his actions in light of his oppressive situation, suggest to Christian Pacifists that non-violence is the model we are intended to follow.
Now finally to the core of your question...
How does a Pacifist respond to 'rampant evil?'
Quoting the article from The Mennonite again (emphasis mine):
Pacifism assumes that punishment and retribution do little to bring about change and tend to escalate the cycle of violence. It is not moral or even possible to “kill all the bad guys.” It is only possible to identify and address factors leading to violence in the environment, such as unemployment. As a pacifist there are causes for which I am prepared to die but none I can imagine that would lead me to kill.
The four principle verses mentioned above suggest directly that a Christian's response to aggression and evil ought to not be violent, and they imply (and it is elsewhere in scripture spelled out) that they ought to trust in God for liberation from evil.
A Christian Pacifist will be willing to die for a cause, but not be willing to kill for a cause.
But does it work?
This is a common criticism... "But if the US had responded with pacifism to Hitler... we'd all be speaking German now!"
We have no way of knowing that, because we did not have a large-scale pacifist response to Hitler. If we had, the world today would undoubtedly be much different. Would we all be speaking German? Probably not. But even if we were, according to a Christian Pacifist, we might be living under oppression, but we would be living free from the guilt of murdering hundreds of thousands of Nazi and Japanese soldiers in the 1940s, and hundreds of thousands of civilian bystanders.
A Christian Pacifist may "lose" in the eyes of the world, and be conquered, but they will uphold a higher standard, and will have won in the eyes of God. "Blessed are the peacemakers."
Even though the Christian Pacifist will say that "even in military defeat we have won", this does not mean that pacifism doesn't work in the world's eyes, as well. There are many examples where it clearly has worked. Look at the independence of India, led by Gandhi, after WWII. A few Indians lost their lives in some peaceful protests, where the British responded with overwhelming violence. But without a doubt, fewer Indians (to say nothing of the British!) died as the result of a non-violent revolution than would have died if the Indians had taken up arms against their oppressors.