@Radu
I personally think that the world leaders are strong willed enough to resist Vader (*waits for someone to rant on about how stupid today's leaders are*), but who knows? However, the world leaders won't let Vader speak to them in any way and prevent all communications.
@Slash
Right...because they have A bombs...
Here's how this works out. The star destroyers show up and america attempts negogiations. We send a few troops in without declaring war and america's economy experiences a small dip. Rumors spread about a recess and people mass-complaign about a depression. We decide that it is racism to attack out of space people. The other nations of the world have the balls to attack but are dominated, and eventually we are subject to the empire's rule and the president surrenders because he's a pussy.