If some nutcase US citizen decides he's going to walk out one morning and punch the first person whose looks he doesn't like, I suspect he would succeed in that. Or any nimrod anywhere. The random belligerant, terrorist, whatever, is going to be hard to stop. The increased security will slow it but we can't all walk around with guns ready to be lifted and fired. And even if we did, you don't know when the nutcase is going to strike. Such things occur so rarely that you can never be completely on guard for them.

No easy answer.

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