A superior's duty to the people under him or her came from "the imperial or the territorial law." Only a "crazy mother" would not defend her child from a dog or a wolf. Christ "did not abrogate this duty, but rather confirmed it."
"Similarly, if a pious citizen sees violence and harm being done to his neighbor, he should help to defend and protect him. This is secular
business, all of which Christ has not forbidden but confirmed."
In short, Luther did not imagine, at least in earthly world before the end of time, some utopia free of violence. To the contrary, he recognized that violence (from wolves and from human predators) existed, and he insisted that good Christians had a duty to use force to defend their neighbors against such violence.
While David Kopel applies this simply to the right to keep and bear arms in Wisconsin, I can see its application on a broader scale, as well.
I've seen, over and over again, the anti-war folks saying that "Bush should have, as a Christian, turned the other cheek" when we were attacked. My reply has always been, "But it wasn't Bush who was attacked, it was three thousand innocent people the POTUS was charged with protecting." As Kopel puts it, "[W]hat is indisputable about Luther is his belief that good Christians sometimes had an affirmtive duty to use violence--in defense of others, in just wars, and in resistance to tyranny." And there are millions of innocents our armies can and therefore should protect, as our duty -- Christian or otherwise.