Some people cite the argument, "We must not preemptively attack someone, or else others will preemptively attack us".
Preemptive attacks are attacks which are designed to prevent someone from hurting you, before they actually do it. It's defensive: it's only done when there's a concrete threat.
First of all, to say we shouldn't do preemptive attacks means that we must wait for someone to hurt us before we can defend against them.
Second, to attack us using the justification of preemption is ridiculous. We don't just go around attacking people for no reason. We're not a threat. If people deal with dangerous threats, that's good. To think that we're a dangerous threat is crazy.
"Some people cite the argument, "We must not preemptively attack someone, or else others will preemptively attack us"."
It is another way of saying, "no one will trust us to keep the peace thus leading others to preemptively attack us".
Putting it another way:
If you knew someone was definitely going to attack you as soon as they got the chance (in the near future), you might want to attack them first to minimise your own losses.
"It's defensive"
Noooooo. By all definitions of offensive tactics, preemptive attacks are offensive.
http://en.wikipedia.org/wiki/List_of_military_tactics#Offensive_tactics
(scroll down)
"it's only done when there's a concrete threat."
Depends what you mean by a concrete threat. If you mean concrete threat to your nation, then no. You don't need to have a concrete threat to have a preemptive attack.
For example:
If the US without warning sent bombers over Canada to take out military targets; the US would have preemptively attacked Canada. Now say Canada is barely a military threat to US and has shown no signs of aggression towards the US. This would be a perfect example of a preemptive attack on a non-threatening target.
Actually ignore my last point about, "only done when there's a concrete threat."