Heavy water, or deuterium oxide (D2O or 2H2O), was an essential compound for the successful production of an atomic bomb. Heavy water was the key material needed for Adolf Hitler's Nazi Germany to build an atomic bomb. However, he was prevented from finishing the bomb after the production facility it was being built in was raided by British Special Forces in 1942.
In an alternate timeline created by the continued living of Edith Keeler, Germany was permitted to complete its heavy water experiments, allowing them to be first in developing of the atomic bomb. This, together with the V-2 rocket, enabled Germany to capture the world, ultimately erasing the Federation. (TOS: "The City on the Edge of Forever")