It boils down to the necessity of making a choice and taking a stand once it becomes clear that the perfect is not an option and that in most cases we find ourselves scrambling to salvage the least imperfect outcome. This is why spiritual sightedness and honesty are so important for the warrior, the statesman, or anyone who must take decisive action when confronted with so many unpleasant and potentially unrighteous choices.
The priest or the academic, for all their lofty achievements, do not really have to face this mess. The monk can turn the other cheek because in the end he is absolved of responsibility to anyone but God, and we can assume that this is pleasing to God. But the same is not true of the policeman, the solider, and the parent. To these, God has entrusted the care of a whole society, city, or household, and when confronted with a violent evil, to turn the other cheek out of some lofty sentiment would be to betray their vocation. The monk sacrifices himself when he turns the other cheek; the father would sacrifice his whole family. The two situations are not comparable and to apply the same moral calculus to each is both cruel and superficial.
The skill that must be developed in the man who resists evil by force is not a willingness to endure evil, but rather the ability to discern between the unachievable ideal and the immediately salvageable good. This is not the same as turning to the lesser evil out of laziness or moral indifference: it is a matter of acting in a context of imperfection.
What the warrior must find in every situation is the actual best, as opposed to the abstract best. Being called to action, he realizes the good here and now and not according to a theoretical formula, and therefore the good he actualizes through his choices will always be a relative good.