The history of Earth-21 is thought to be largely similar to that of Earth-Two, including the arrival of Superman from Krypton in the earliest part of the 20th century, the deaths of the Waynes in the 1920's and the arrival of Wonder Woman to Man's World in the 1940's.  The JSA is known to exist and known members are largely identical to Earth-Two tho the full roster may have not been revealed. 

World War II largely unfolded as it had in other timelines.  In 1945, a military stroke force known as The Losers happened upon an island where Rick Flagg is in need of rescue.  Assaulted by dinosaurs, the Losers rescue Flagg but are all killed, leaving the message the island is actually alive.  After the war, public sentiment becomes paranoid and most costumed vigilantes are forced to retire and the JSA disbands.  Whether it ever re-forms is unknown.  Some heroes, like Superman and Wonder Woman operate under government sanctioned programs but many like Batman, continue to operate in the shadows.  When Hourman attempts to stay active, he is tracked by a government operation headed by King Faraday and is believed killed in the process.

Over the ensuing decades, a number of events unfold.  Wonder Woman becomes a gov't agent in Indochina but puts her own spin on things, liberating women in the process.  Superman and Batman stage a fight in which Batman wins allowing them to operate and Superman, Batman and Wonder Woman meet to ultimately form an alliance that guides expectations of costumed vigilantes.

In the late 1950's, events unfold that are best known on Earth-One.  Barry Allen is struck by lightning amidst chemicals and becomes the Flash.  J'onn Jon'zz arrives on Earth through the machinations of Dr. Erdel and becomes trapped on Earth-21. Green Lantern is contact by a dying Abin Sur to become the sector's newest Green Lantern.  Unique events to this timeline are the risk of a Black hero who takes the name John Henry after his family is murdered by the Ku Klux Klan.

Jon'zz adopts the identity of a Gotham detective and begins to follow what he sees as patterns of alien influence.  He terms it The Centre and by the late 1950's, the influence in enormous.  Demoralized by Faraday's constant torment of heroes, he decides to leave and turns his notes over to Batman.  Batman, realizing the scope of the problem, passes the baton to Superman.  The Centre attacks Themyscira, badly injuring Wonder Woman, who leaves to warn American superheroes. 

When the Centre arrives off the East Coast, a mass of costumed adventurers gather in response.  Superman attempts to probe the Centre and disappears, galvanzing the others into actions.  After a pitched battle, the Flash uses Ray Palmer's white dwarf technology to shrink it small enough and unstable enough for Green Lantern to use his power ring and fling it into space where it is destroyed.  Aquaman emerges from the sea carrying Superman who ha survived after all.  The assembled heroes then usher in the next generation of super-heroes on Earth 21 (DC: The New Frontier LS).