Watchmen takes place in an alternate America of 1985, complete with Cold War references like the Doomsday plot and nuclear paranoia, along with more subtle changes. The history of Watchmen's world mirrors our own until the 1940s, when "costumed adventurers" - the book's name for superheroes - started to appear (This timeline follows our world's timeline for the emergence of superheroes in comics). In 1959, an accident transformed scientist Jon Osterman into Dr. Manhattan, a being with godlike power, permanently shifting the global power structure in the US' favor, leading to (amongst other things) Vietnam becoming the 51st state of the USA following a significantly different war in the 1960s. In Watchmen's 1985, Richard Nixon is still president, having used the Comedian to cover up Watergate successfully and repealed the 22nd Amendment to remain in power for five terms; the result, perhaps, of Alan Moore's own political pessimism about then-British Prime Minister Margaret Thatcher's successful third election.
Amongst the minor, yet interesting, changes in the world in this alternate timeline: Superheroes are, essentially, outlawed by a 1977 law introduced by Senator Keene, which banned "costumed vigilantism" as a result of riots following the police going on strike in protest to extreme methods used by superheroes. The few active superheroes remaining are either outlaws (Rorschach) or government agents (the Comedian); this idea was later recycled by Marvel Comics for their 2006 Civil War series and related comics.