A little background on what scientists know about atoms at this time:
Everything in the universe is made up of tiny particles, called atoms;
atoms are made up of even smaller particles: a nucleus composed of protons and neutrons packed tightly together, and then electrons orbit around the nucleus;
some atoms are radioactive, i.e.: their nucleus is unstable, which means that particles can break away and shoot out at high speeds.
So. It's 1938, and this guy Otto Hahn, a German chemist, sets up an experiment to see what will happen when a speeding neutron from a radioactive element crashes into a uranium atom.
Hahn discovers that the force of the collision causes the uranium atoms to split in two. This just about blows his mind.
Hahn immediately reaches out to his former partner, Lise Meitner, a German physicist, and asks what the heck this all means.
She doesn't really know what it means, so she asks her nephew Otto Frisch—also a physicist—and he's befuddled as well.
The two of them (Frisch and Meitner) begin really contemplating this uranium problem, and they realize that a theory of Niels Bohr's (about atoms being like water droplets) might explain everything.
Then, Meitner and Frisch realize that if an atom splits in this way, it will also produce a lot of energy as it breaks in two. So, if you were to split a whole bunch of uranium atoms at the same time, you would be releasing a whole poop-ton of energy—you'd essentially have the most powerful bomb ever built, by a long shot.
This news spreads like wildfire through the theoretical physics community. When Frisch tells Niels Bohr what they've discovered, he basically slaps his forehead and goes, "Well, duh."
He immediately shares his calculations (which he frantically does after speaking with Frisch) at a physics conference in Washington, D.C., and then newspapers publish his findings all over the country.
A young physicist named Luis Alvarez stumbles upon one of the blurbs and gets up, mid-haircut, to run the news to Oppenheimer.
Oppenheimer gets all fired up by the possibilities uranium fission could have, including using it to create a weapon of mass destruction.
However, Oppenheimer—being the smart dude that he is—quickly realizes that if his first instinct is to use this new power to make a huge bomb, he's not the only one having the same exact thoughts. This ordinarily wouldn't be a bad thing, but since it's the beginning of World War II, and the Germans are the ones who made the initial discovery…Yeah, this could be problematic.