Used by the Romulans in TOS: The Balance of Terror.Potato ChipA photon torpedo, when set at higher yeilds, is more powerful than a nuke.Given a statement in Star Trek the original series one could conclude that the missile weapons utilized by NX-1 where atomic in nature
Actually, the weapon used by the Romulans in the TOS episode "Balance of Terror" was a Plasma Torpedo. You can find information on that here: http://memory-alpha.wikia.com/wiki/Plasma_torpedo.
In the Star Trek: The Original Series episode "City on the Edge of Forever" it is explained that the pivotal event in history that McCoy's intervention ensured was that Nazi Germany was able to complete its "heavy water experiments". Germany "captured the world" when it developed the first nuclear weapons, rather than the Allies having developed them first, as in the normal universe. Deuterium, of which heavy water is mostly comprised, continues to be important into modern Star Trek, as deuterium is the matter reactant in M/AM reactors powering warp drive.
It also stands to reason that a pivotal moment in the development of the Mirror Universe had to have been Nazi Germany developing nukes first. Probably, also, was the Confederacy winning the U.S. Civil War. Note the overt racism mentioned in ST:ENT "In A Mirror, Darkly". Nazism remains, to this day, one of the scariest, if not the scariest, development in Human history.
Canada manufactured an abundance of heavy water in order for the Allies to develop nuclear weapons first, resulting in Humanity's first detonation of a nuclear device at the Trinity test near Socorro, N.M. on July 16, 1945. A Canadian scientist was also directly involved in the Manhattan Project. Germany had a nuclear weapons program; however, it did not have a reliable source of heavy water, and therefore it still had not completed its heavy water experiments, and had no functioning reactors or usable fissile materials, and had surrendered to the Allies, by the time the Allies nuked Hiroshima on August 6, 1945, and Nagasaki, on August 9, 1945.
Ellison and those writers who came after him were probably correct--that the side that developed nuclear weapons first would be the prevailing side of Human history. Put that together with a prevailing economic theory, and what results is American hegemony.