(covers information from several alternate timelines)
The decade of the 1940s on Earth is marked by World War II. Germany under Nazi rule has fronts against England and the USSR. With the attack on Pearl Harbor, the USA enters the War from which they emerge victorious and are established as a world superpower for centuries to come. Harry S. Truman becomes president after the death of Franklin D. Roosevelt and remains in office during the decade. In two alternate timelines, the outcome is radically different, with Nazi Germany conquering much of the world.
Not surprisingly, scientific breakthroughs on Earth are centered on warfare, such as heavy water experiments, the atomic bomb (utilizing the principles of nuclear fission and/or nuclear fusion), and the V-2 rocket.
In the field of art and entertainment, the films For Whom the Bell Tolls and Abbott and Costello Meet Frankenstein are released, along with stories featuring Dixon Hill. Songs released in this era include Moonlight Becomes You, How High the Moon, My Old Flame, Out of Nowhere, Let's Get Away from It All, It Can't Be Wrong, That Old Black Magic, and You're Nobody 'til Somebody Loves You.
- World War II rages on Earth. (TOS: "The City on the Edge of Forever"; VOY: "The Killing Game", "The Killing Game, Part II"; ENT: "Storm Front", "Storm Front, Part II"; et al.)
- In an alternate timeline, Edith Keeler is saved from the traffic accident by Leonard McCoy, allowing her to continue striving for her goals of world peace, which ultimately delays the the United States of America' entry in World War II. As a result of these actions, Germany is allowed to complete its heavy water experiments and this enables them to be first in developing of the atomic bomb. This, together with the V-2 rocket, enables Germany to capture the world. (TOS: "The City on the Edge of Forever")