Thanks Adrian! Wonderfully interesting!
There are, as deb points out, some subtle and very interesting differences between the two prefixes. It does clarify things to note how the McCarthy period used the *un* prefix (a weird and repugnant totalitarian-style notion of proper nationalist sentiment/idea). I confess I was unaware of the history in Australia of such usage. I am pondering now what reflection of the same thing might be present in my country.
But again, as deb suggests, the *anti* prefix does a somewhat different, or additional, bit of work...implying some dangerous and threatening external agency. Deb uses the term *paranoid* and, in the case of the US, the term is not necessarily misplaced (see
HERE)
The clearest analogy I can think of to this American hyper-sensitivity or misplaced sense of imminent victimhood would be Israel. But of course, the differences in actual threat are magnitudes greater for Israel.
Or take a comparison with Canada. Very likely, Canadianism (whatever the hell that might be conceived as representing) is threatened by our big neighbor and its cultural/political power far more than America might be threatened by anything real, simply as a consequence of scale. This is also the French-Canadian complaint...that English Canada (and the English US) pose threats to French-Canadian identity as a consequence of scale. But what sort of comparable threat exists for America? Well, everybody else! And that gets us to the wonderful satire in the Randy Newman song Political Fiction...no one likes us so lets drop the big one and see what happens (we will show them!).
So it seems that the frequent and passionate use of *anti-Americanism* is functioning in some manner quite different from what most users suppose.