Oppenheimer is haunted by the media attention he's getting now that the bomb is all over the news. Sure, being famous is fun, but not when you're famous for finding the newest way to kill a bunch of people all at once.
Oppie starts dedicating himself to a new quest: to stop the U.S. and other countries (ahem—looking at you, USSR) from building more atomic bombs.
Groves and Truman think Oppie's crazy for wanting them to stop. They want more bombs—lots and lots of bombs.
The scientists at Los Alamos, feeling they've achieved their mission and would like to get back to their normal lives, become pretty disenchanted with building atomic bombs.
Oppenheimer resigns as director of Los Alamos in October of 1945.
He's basically obsessed with the prospect that the human race is on a path to destroy itself using the weapon he helped create.
Oppenheimer meets with Truman to try to prevent an arms race with the Soviets, but Truman is the very definition of hubris and the meeting is a failure.