There is no 'alien' invasion.
Even as far back as the horrible Sam Hamm script effort of the late 80s, NO professional scripter has included this ending in a Watchmen movie draft, including David Hayter.
And that includes current writer Alex Tse. His draft dated Nov. 10 2006 which is on its way to being the one filmed incorporates the exact same ending as his very first attempt from summer last year.
Which is: Veidt unleashes a deadly energy ray into the atmosphere from his Antarctic fortress, synthesized from Dr Manhattan himself. This strikes down back to earth in the form of blue lightning, hitting several key cities worldwide (no longer just NYC). In total, seven million people are vaporized on the spot as a result. The way in which its done by Veidt is designed to frame Dr Manhattan, making mankind believe that Big Blue is responsible for this devastation having finally had enough of our warlike ways. The idea behind this deception is to fool our leaders into thinking that Manhattan will always be watching over us henceforth, so therefore play nice as a race from now... or else.
There you go. Not a gloopy tentacle in sight.
Hollywood is NOT going to show us a city clogged with twisted corpses and rivers of blood flowing in the streets, folks, no matter WHAT the rating.
Did you seriously believe for one minute they would...?