Say you are a pacifist and the first thing someone will do is throw a hypothetical self-defense or defense of innocents situation at you. "What if a robber broke into your house and threatened to kill your wife and children..."
What this reveals is that for the most part violence is already understood to be illegitimate by most people in most circumstances. Very few are willing to argue in favor of unilateral aggression, or retributive violence. The difference between a pacifist and most people is that a pacifist says, " even in the case of violent threats I choose to refrain from responding violently." So we are not arguing about 99% of violent scenarios. We are arguing about 1%.
I could go into detail about various hypothetical scenarios, or historical scenarios arguing from a pacifist viewpoint. I could point out (as I have in the past) why these scenarios are dishonest, but here I want to make a different point about why it is crucial to give up on self-defense even in that vanishingly rare hypothetical situation. The reason is this: self-defense inevitably becomes a cover for other types of violence.
Think about the wars the US has waged in the past century. Which one of them was not justified on the basis of self-defense? Even in the most extreme cases of obvious unilateral aggression such as the most recent Iraq War the justification was "weapons of mass-destruction", "Iraq-AlQaeda link", "defending Iraqi people from an evil dictator" etc... The doctrine of pre-emptive warfare is just self-defense stretched beyond the breaking point. WWI? Self-defense. WWII? Self-defense. Korean War? Self-defense? Vietnam? Self-defense. We've been defending ourselves from Nazis, communists, and terrorists over and over again by spending billions of dollars to fly overseas and bomb other countries.
In the case of an individual - when am I entitled to defend myself with violence? When someone physically attacks me? When they threaten me with a gun? When they loom over me menacingly? When they verbally abuse me? Is me feeling scared a sufficient justification for violent response? How can I even make a wise decision about proportional response to danger when I am in a state of terror? It seems like begging for a tragedy to me.
As long as we keep self-defense on the table it will continue to be a universal justification for any kind of violence that we can pretend falls under that umbrella. We adamantly refuse to see ourselves as anything other than paragons of nobility and peaceful intentions beset by violent bullies on all sides. If you want peace the only option is to give up the ways of war.
Speaking theologically I think this is a reason we need to cling to some language about Christ that is repugnant to some feminists and liberation theologians. To understand Christ as submitting himself to evil, or as a willing sacrifice may indeed be problematic from the perspective of victims of violence and the oppressed. It certainly has been abused by those in power to encourage passive acceptance of injustice. I understand this critique - but is there anything more urgent for those who walk the corridors of power or for any budding peacemaker to grasp than the truth that we absolutely cannot, under any circumstances, return evil for evil? Is it not close to the heart of the gospel to say that we must endure calamitous injustice rather than lift a finger in retribution? Isn't that what Christ actually does on Good Friday and Easter?