Al Quaeda didn't make the Russians and Americans team up to wipe out Islam. So why would an alien attack?
Well Al-Qaida only attacked the US, it did unite Americans until the media decided division sells. The theory is that if we had a common enemy we would be united. But as you say if a bomb went off in Moscow especially at that time, they aint gunna wait to see if Manhattan went rogue, they're gunna nuke us.
Maybe I'm wrong, but I thought-in the book-the only site that was destroyed was a portion of New York. That's why I drew the Al Quaeda comparison.
Like 9/11, in Watchmen (the book) the US was attacked by an outside force that posed a threat to the entire world. Unlike 9/11, in Watchmen that resulted in the US and USSR putting aside their differences.
I think the big difference there is that the alien attack was sposed to suggest that the whole world would believe the aliens would not differncitate between one country and another, where as with Al-Qaida, not every country would view them as a threat to them, plus the knowledge of this terrorist organisation was not something unknown to them where as the alien came totally out of left field.
I think the alien being uniting the world was somewhat idealistic, but to me, made more sense than the Manhattan devastation. As you say, the first thoughts going through the Russians heads during a time of potential war would not be "Oh, its Doctor Manhattan" or "How many other countries have been hit", it would be "Fucking Americans, launch a counterstrike!"