> I figure US invades Canada to be least likely. These are two very stable nations. The US would have to be facing some huuuuge crisis in energy supply, global warming, or overcrowding, with Canada holding the key to one of these somehow. I hardly see it happening, even in fifty years, but you never know.

Canada has 60% of the world's reserves of drinking water.

The U.S.A. is wasting water and an exponantially increasing rate.

Plus:
We have as many weapons of mass destruction as Iraq, and the chances of finding either Ossama or Saddam seem to be the same if you invade Afghanistan, Iraq, or Canada.

So, likelyness == "high"

You can't take the sky from me...