This decade on Earth was marked by World War II. Germany under Nazi rule had fronts against England and the USSR. With the attack on Pearl Harbor, the USA entered the War from which they emerged victorious and were established as a world superpower for centuries to come. Harry S. Truman became president after the death of Franklin D. Roosevelt and remained in office during the decade. In two alternate timelines, the outcome was 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 were released, along with stories featuring Dixon Hill. Songs written 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 was allowed to complete its heavy water experiments and permitted them to be first in developing of the atomic bomb. This, together with the V-2 rocket, enabled Germany to capture the world. (TOS: "The City on the Edge of Forever")