If you’re going to watch the movie in preparation for the HBO series, there’s at least one big difference between the comic and the movie you should know about:
At the climax of the movie, Doctor Manhattan is framed for a horrifying attack that levels New York City, killing millions of people. That’s not how it goes down in original. In the comic, the attack that levels New York City appears to have been caused—but was not actually caused—by a giant alien squid that slips through a trans-dimensional portal.
The "alien attack"—and the implication that the Earth might be in more danger from an extraterrestrial force—is enough to scare both the United States and the Soviet Union away from the brink of nuclear apocalypse, banding together against a greater alien threat.
Which, as it turns out, was the goal all along. Because the "alien attack" was actually an elaborate hoax engineered by a billionaire businessman and philanthropist named Adrian Veidt—also known as Ozymandias—who will be played by Jeremy Irons in the HBO series.
The comic begins with a series of mysteries, and eventually reveals that Adrian Veidt is the culprit behind most of them. Before the events of the comic, Veidt had analyzed the geopolitical landscape and concluded that the United States and Soviet Union were on a path that would lead to the destruction of the entire world. So he secretly used his vast resources to fake the alien attack, concluding that the millions of people who died in New York were a necessary sacrifice for the greater good. And whether you agree with Veidt’s method or not, it worked.