It doesn't SEEM hard, which is exactly why it's rather insidious. What does Mustard Gas REALLY have in common with a nuclear bomb? If the latter is justification for an attack, is the former too, by association? If not, then the WMD category is not a useful category for the argument on whether to go to war with Iraq. But because it seems reasonable in other contexts, people accept it as a natural category...

Sorry. Off soap box now....