Stories set in an alternate history or reality are built from a "point of divergence," a moment at which the fictional reality veers off from our own. Germany wins World War II, Kennedy survives the assassination attempt, etc. In Watchmen that point comes in 1938. Shortly after the publication of Action Comics #1, costumed heroes begin appearing in the real world, the "factual black and white of the headlines," as Hollis Mason puts it, and history changes course.
In our reality, comics books experienced their own point of divergence on June 5, 1986, with the debut of the first issue of Watchmen by Alan Moore, Dave Gibbons, and John Higgins. Ever since then, the entire medium has been permanently altered by its startling vision and precise execution.