The Vergeltungswaffe 2 (German for Vengeance Weapon 2), or V-2 rocket was a ballistic missile developed by the Nazi German state on Earth in the first part of the 20th century. Originally known as the A-4, during testing this rocket was the first manmade object to leave the Earth and enter space by reaching a altitude of 189 kilometers. Many of the V-2s were built using slave labor - namely concentration camp prisoners, such as humans of the Jewish faith. The V-2 was used later in World War II by the Nazis against targets in Belgium and Great Britain.
Following the war, much of the technology wound up the hands of the United States of America and the Soviet Union. Wernher von Braun and Walter Dornberger were among the designers who were in American hands following the war. The V-2 rockets were used as a foundation of the later 20th century space programs. (Historical accounts)
In the alternate timeline created by Doctor Leonard McCoy, the United States did not enter the war until much later. This gave Adolf Hitler the time he needed to develop nuclear weapons, and he was able to use them in conjunction with the V-2 rocket to conquer Earth. (TOS episode: "The City on the Edge of Forever")