But beyond this simple fact that the Christian must base their morality on Christ's teachings rather than on what is convenient, I think there is a simple answer to the question posed here. Yes, I as a pacifist should not harm other human beings. But I don't have to worry about other Christians who are following Christ's teachings. We are in his Kingdom and we are choosing not to do evil. So if the United States is attacked, it won't be by Christians (or at least Christians who embrace Christ's teachings on non-violence). Where should violence be coming from? It should be coming from those who are not Christians. It's coming from the world. As a Christian, I have done my part to combat evil by spreading the gospel and pacifying a portion of the world to violence. That is the Christian way. But those who are secular do not have the same ethic as Christ's ethic. Individuals who are instigating violence are not pacifistic Christians. So let the world take care of its own problems with its own means. Yes, this may mean that when those from the world invade our lives and attack us, we do not retaliate with violence. But that is not our fight, it is God's fight, and his means is to use his sovereignty over governments to deal with evil by using the sword.
Obviously, under this idea, were a nation to become largely pacifistic, then they would not have the ability to fight violence with violence. But first, this is why pacifists tend to view the government as something left to the world - particularly positions which would require compromise with Christian values (the president is commander in chief of the army, the military, congress because they need to make decisions about war, etc). Pacifistic Christians tend to recuse themselves of governmental roles that would require a split allegiance. The government's role is to look out for its own interests. That is not even close to the Christian's call. Therefore, as a Christian, I should abstain from positions of government that would cause me to betray one of my commitments. Second, when would any nation ever become 100% Christian? If there remain secular individuals in society, than the government can be run in a secular way and can wage wars if it wants. As the early church advocated, Christians can pray for these wars and be the conscience at home, but we are not to do harm to another.
Many Christians have problems with this because they are think that abstention from significant governmental roles is 1) avoiding our responsibility, and 2) not leveraging an important power structure for Christian influence. To these accusations I would argue that this is a misrepresentation of what pacifists argue. Pacifists are not saying that we avoid action, but rather that we focus our action through a different means. God may be sovereign over governmental institutions, but he conquers hearts and the world through his church who uses the means of love. Governments legislate. The body of Christ lived as he commanded reforms. So instead of spending countless dollars and hours lobbying for laws on abortion, pacifists are more likely to advocate for actions that change lives by directly enacting good and by touching the hearts of others though example rather than words, laws, and force. Pacifism isn't shirking responsibility in refusing to leverage government for Christ. Rather, it views the lever of power, the means of God, as being something altogether different. the Kingdom is lived out through the church and it is this community living in love that bears witness to our true allegiance, as Jesus himself says. It is also through this Kingdom that true change will come.
It is not the Christian's job to ensure the outcome of events. It is our job to submit in obedience to our king. We must understand that God's means are always better. Sometimes they're better because they're more effective. Love and sacrifice may change hearts whereas war and violence may create temporary peace only to embitter nations against each other for centuries. Sometimes God's ways are better because our obedience and faitfulness to God causes him to respond with protection and provision in return. Though immediate temporal blessing isn't always linked to moral actions, sometimes it is. Sometimes God's ways are better not because they provide temporal provisions, but because following God's means are eternally more rewarding, both in terms of the souls who see God through our faithfulness and come to know him, and in our storing of our own treasures in heaven. So yes, we are to seek the welfare of our cities and nations. But to act as though seeking the welfare means pulling on whatever levers of power we must to ensure the outcome is just wrong. If non-violence is the means God has called us to, the metric of ensuring ends is not a metric of God. Faithfulness is. Yoder, in "The Politics of Jesus," summarizes this concept well.
Whether Jesus be the Christ or not, whether Jesus the Christ be Lord or not, whether this kind of religious language be meaningful or not, most types of ethical approach will keep on functioning just the same... The cross is not a recipe for resurrection. Suffering is not a tool to make people come around, nor a good in itself. But the kind of faithfulness that is willing to accept evident defeat rather than complicity with evil is, by virtue of its conformity with what happens to God when he works among us, aligned with the ultimate triumph of the Lamb.
The vision of ultimate good being determined by faithfulness and not by results is the point where we moderns get off. We confuse the kind of 'triumph of the good,' whose sole guarantee is the resurrection and the promise of the eternal glory of the Lamb, which an immediately accessible triumph which can be manipulated, just past the next social action campaign, by getting hold of society as a whole at the top. What in the Middle Ages was done by Roman Christianity or Islam is now being attempted by Marxism and by democratic nationalism. In spite of all the difference in language, and in the detailed vision of just what a good society would look like (and as a matter of fact even the visions are not that different), the real uniqueness of each of these positions is only that it identifies differently the particular moral elite which it holds to be worthy of guiding its society from the top. We may well prefer a democratically controlled oligarchy to some other kind. We may well have a choice between Marxist and Islamic and other statements of the vision of the good society. But what our contemporaries find themselves practically incapable of challenging is that the social problem can be solved by determining which aristocrats are morally justified, by virtue of their better ideology, to use the power of society from the top so as to lead the whole system in their direction.
Once a desirable course of history has been labeled, once we know what the right cause is, then it is further assumed that we should be willing to sacrifice for it; sacrifice not only to our own values but also those of the neighbor and especially the enemy. In other words, the achievement of the good cause, the implementation in history of the changes we have determined to be desirable, creates a new autonomous ethical value, 'relevance' itself a good in the name of which evil may be done.