To spell that last point out: Jesus never advocated that his followers use violence to defend themselves. The ethics of the Sermon on the Mount are quite pacifist, esp. Matthew 5.39, "If someone strikes you on the right cheek, turn [to him] the other one too," as well as some of the Beatitudes.

Of course Jesus never thought that his followers would have to worry about living life in this world for very long, since the Son of Man was going to come soon. But he was wrong about that; and esp. since the time of Constantine, Christians have to worry about running governments. In that case, it is a noble and saintly course of action to follow, to be a pacifist; on the other hand, when the safety of innocent people is our responsibility, pacifism begins to look sinful.

To be sure, the militarism of US policy and culture is far beyond what straightforward defense calls for, and looks plenty sinful. And Berrigan, Dorothy Day, and Thomas Merton were right to protest as they did.