The shootings in Norway, like the attacks on 9/11, are a reminder that we're our own last line of defense

For most people in the United States, Canada, and Western Europe, self-defense needn't be a daily preoccupation. It is rare that someone is murdered by a stranger. Insured against robbery, banks instruct their tellers to hand over cash if they get so much as a threatening note. On city streets muggers are told, "Take what you like, just don't hurt me." In suburbs, families who return home to signs of a prowler sit a few doors down in their cars and call for law enforcement. We've unburdened ourselves. The average citizen need never defend the city walls, or join a posse to pursue a horse thief, or patrol his neighborhood. Even wars are now fought by professional soldiers in all-volunteer armies.

So we forget. That there isn't always someone to call. That sometimes we're confronted by horrors even if we didn't volunteer for them. That we each therefore bear ultimate responsibility for defending ourselves and our communities.

It is our inescapable burden.

The people of Littleton, Colorado learned that lesson when two Columbine High School students went on their rampage. On 9/11, the passengers on Flight 93 understood it in real time, and heroically marked the beginning of a new American era in air travel: never again are passengers going to submit to a hijacking without fighting back. The victims of Virginia Tech were confronted with this hard truth too. All they could do to defend themselves was barricade doors.

As a society, we can only do so much to stop unhinged killers. Humanity is unlikely ever to be rid of them. Sometimes they'll succeed in their murderous enterprises.