Watchmen is first and foremost a work of fiction, and so can take as many liberties with history as it wants. That being said, the internal history of the story contains some interesting extrapolations based on what might have happened had a truly Omnipotent super being existed during the Vietnam War. Dr. Manhattan singlehandedly defeats the VietCong, even if he does not win a war of goodwill. America's victory allows Richard Nixon to remain in power. The slump of Ford and Carter never happens, and so presumably the Reagan era never occurs either.
Another topic is that of moral choice. The character Rorshach was created by Moore as a parody of Steve Ditkos's Mr. A, an Objectivist superhero, and is very extreme in his thinking. Rorshach refuses to compromise; he will not reason with his victims and has no qualms about killing those he believes to be morally inferior or ethically incorrect. In the end, he confronts his beliefs up to the point of death, and although he does not compromise, he shows at least some signs of remorse or understanding. His character is in some ways the most honest in the book; he does not change his opinions for anyone or anything.