Hm... I love Watchmen, but I'd like to point out that the original alien ending isn't exactly bulletproof. It's built on the notion that the smartest man in the world comes up with a way to end war by uniting the world against a common threat... but the way he did it, the peace could only be fleeting at best. I mean, the world would be united, but against what? Against aliens? The threat is non-existent. The world has no explanation of how it came about, so they have nothing to fight, and the lack of follow-up attacks would completely dissolve the perceived urgency of the situation.
Okay, one big, unexplained alien kills millions, and years go by without any sort of repeat occurrence... and back to your regularly scheduled programming.
I mean, I don't want to presume to be smarter than Alan Moore, so if anyone has an explanation of how the "alien attack" is supposed to be an effective long-term solution, please explain.