A useful unifying principle for action is that we should always favor the minimal amount of violence.
Pacifism is the belief that one should never use violence and pacifism is open to a number of criticisms. First it is possible to set up thought experiments that clearly challenge the ethical basis of pacifism; would you not use violence to prevent a person from causing harm to a child? Or to hundreds of children? More than just theoretical, there are historical examples, such as using force to overthrow slavery in the American south, that have strong justification. So I will start with the assumption that absolute pacifism is untenable.
However, a more modest proposal seems in order, we should always strive to use violence only as a last resort and when force is necessary we must always try to apply the minimum amount. Many people would assent to this proposition, but we often stray from it in application. We have a tendency to adjust downward the point at which violence might be necessary. After the September 11th attacks many of us were ready to endorse all kinds of ill conceived mobilizations of violence. In retrospect we can see how poorly conceived our interventions in Afghanistan and Iraq were. These examples make it clear that we need to adjust upward the point at which we think violence is justified.