In the series Oliver Stone's Untold History of the US he says that the first splitting of a Uranium atom made an atomic weapon a plausibility. But why was Uranium first decided as an atomic necessity?
This question seems a little confused; it is a science question, not a history question. Nuclear weapons only work because of nuclear fission (the "splitting" process), a process that only takes place in a very small number of elements. The only one of these elements that is found in nature is uranium (and less than 1% of uranium in nature is of the type that can be used in weapons). So that is, understandably, the element in which the phenomena was first observed. There are a few other elements that can be used for this purpose, but they were discovered later (and the main other element used in weapons, plutonium, is made from of uranium). But the reason that it was uranium originally is because that is really one of the only ways to do it.
You should check out Wikipedia's page on nuclear fission, and come back if you have historical questions that it doesn't answer. For scientific questions, try /r/askscience or /r/explainlikeimfive.
There'a a lot to discuss regarding nuclear weapons and 20th century physics here, so I'll start with the basics:
Atoms consist of a positively charged nucleus surrounded by a cloud of negatively charged electrons. The nucleus consists of positively charged protons and neutral neutrons. The number of protons determines the charge of the element and its place on the periodic table.
The best way for scientists to investigate sub-atomic (inside atoms) events is by bombarding atoms with particles and observing the effects.
Before 1932, when the neutron was discovered, physicists had primarily used protons to bombard atomic nuclei. Since both protons and nuclei have a positive charge they repel each other. As a result, protons would miss the nucleus or would be slowed too much to cause any kind of interesting result on impact.
In 1934, shortly after the neutron was discovered, Enrico Fermi (an Italian scientist) was the first to split the atom (although he didn't realize it at the time). He bombarded a Uranium nucleus with neutrons, causing it to split roughly in half into two lighter elements. (Fermi didn't understand that this was happening, he theorized that the neutrons were bonding with the uranium atoms, creating a new heavier element. It wasn't until 1938 that Otto Hahn and Fritz Strassmann determined what was actually happening, which is why they get credit for discovering nuclear fission). Uranium was used in these experiments because it is the heaviest naturally occurring element, and as a result, has the most potential energy contained in its nucleus. When a Uranium nucleus is split, it (most commonly) produces an atom of Krypton, an atom of barium, and several neutrons.
However, splitting the uranium nucleus was just the first step! When a nucleus is split, a tiny amount of its mass is converted to energy and released, approximately 3*10^-11 J. While this is a tiny amount of energy (Fermi once estimated that it was enough to make a grain of sand move slightly), that is the energy released per atom.
In an atomic bomb, the neutrons released during fission cause a chain reaction by splitting adjacent atomic nuclei. Thereby, one split nucleus releases neutrons which split 2 nuclei, then 4, then 8 and so on until they run out of nearby nuclei. Each split releases more energy. This is what causes a nuclear detonation.
Source: Richard Rhodes, The Making of the Atomic Bomb