Whether a Christian can follow the Bible and be a Christian Realist:
Objection 1. Christian realists (especially Reinhold Niebuhr) say yes, in our day and age the only righteous way to be a Christian is as a realist. The Christian Scriptures clearly state that we should be nonresistant to evil, but that these words were written in another time and place and are not meant to be followed literally today. Instead we need to be willing to fight in defense of God's values and God's people.
Objection 2. When the Christian Scriptures were written, Christians were a persecuted minority. Our actions must change now that we have been given the authority of running nations.
Objection 3. To see evil being done and to do nothing about it is worse than breaking God's law of not harming others. We cannot sit idly by and watch injustice happening. It is better to use some evil and ask forgiveness than allow a greater evil to occur.
On the contrary, peace churches across history have upheld the Jesus' command to not use violent force, and to instead actively work for peace in whatever time and place in which they find themselves.
I reply that for Christians, there is no justification for intentionally breaking God's command to not harm others. It may be more "realistic" to fight wars that seem to bring about a good end, but taking this decision into our own hands is attempting to do God's work. Christians are not called to be effective: we are called to be obedient. At the same time, doing what is obedient is often effective in bringing about good ends without recourse to violence. Nonviolent direct action works at least as well as war in bringing about good ends, and usually there is less bloodshed. The realist position assumes that war actually succeeds in bringing about good ends, which is a position which is fairly suspect. War may succeed in stopping some evil from happening, but rarely does it leave a peaceful situation where a community can easily return to a normal, pre-war state. Christian realists assume that in this world where humans are always going to make evil choices, there will always be the necessity to stop them, and this will require the use of violence. I believe that, on the contrary, we are called to live in the Kingdom of God here and now, and make that our witness to the world of what God's Kingdom is like. If we act in the same evil, violent ways as the world we are not bringing the Kingdom of God to Earth, no matter how just our intentions.
Reply to objection 1: If the Christian Scriptures clearly state that Christians are to be nonresistant to evil, but to instead show up evil by staying in the good, who are we to decide when we should go against that teaching? If God told me directly, "You have heard it said that you should no longer use violence, but I say to you that now it's time to fight a holy war again," perhaps I would do so. But I'd have to be pretty sure it was God (we're talking an actual voice from the clouds and miracles, etc.) before I would go against clear biblical teaching. Some things in the Bible do seem cultural and meant for only for a specific time and place, but this doesn't seem like one of them to me.
Reply to objection 2: One question we might ask is whether Christians should be willing to be involved in running nations if it causes them to compromise principles Jesus laid down for us. For Jesus and his early followers, persecution was a way of life not because they lacked the skills for national leadership, but because their teaching was so foreign to the ways of the world that they were ridiculed, seen as a danger to the (unequal and unjust) status quo, and many were persecuted. We might ask ourselves why it is that our brand (or is that "bland"?) of Christianity is so acceptable to our culture. It would be a wonderful thing if Christians actually ran a nation as Jesus asked his followers to live: if we gave food to the hungry, showed active love to our enemies, and lived with the humility of "the last shall be first and the first shall be last." But our country's government (USA) and other governments with a large Christian population do not conduct themselves in this manner, and thus are not following Christ. Christians in power should not act differently from Christians who are a persecuted minority.
Reply to objection 3: With this objection I whole-heartedly agree. This is the single greatest criticism of pacifism, in my opinion. We CANNOT sit idly by and allow injustice to be done while we do nothing to stop it. It IS better to resort to violence in order to stop injustice from being done, rather than to do nothing. But it is even better to do something nonviolent that will diffuse the situation without the loss of life, or that will show the injustice of the situation in a way that the oppressor cannot ignore, even if that requires loss of life. If pacifists are really serious about this belief, we should walk our talk: we should be as willing to lose our physical lives for the sake of bringing forth God's Kingdom as so many are for the sake of defending the honor of their earthly kingdoms.