Not sure about the US, but in the UK you are allowed to take pre-emptive action against someone if you perceive yourself to be in immediate danger (I nearly said "clear & present").
For example, someone holds a knife in front of you, you smack them over the head with an iron bar. That's what happens when you threaten people with knives IMO.
Oh, I agree. But remember, we went to war because we
couldn't see the WMD's. That be like if I hit you over the head with a bar because I
thought you had a knife, but you wouldn't turn out your pockets to prove that you didn't have one. Now, sure, I saw you with a knife, once (sold it to you, in fact) but that was ten years ago...
And now what are we discovering? A distinct lack of a knife on your hypothetical person. Of course we have an excuse - "he ditched it while I was hitting him."
And the precise reason we went to war with Iraq (and not N. Korea) is because we weren't in
immediate danger, but rather, we were in danger of being in immediate danger
in the future. Like, you were
about to threaten me with a knife.