Semyon Semyonov has a legitimate engineering job that gives him a reason to be in the U.S., but the FBI still (correctly) suspects him of being a spy.
He and a KGB buddy, Alexander Feklisov, begin working together to shake their tails (i.e., get rid of the guys following them, not behave like happy canines).
Both Semyonov and Feklisov feel that what they're doing isn't wrong. In fact, they believe that America's assistance to the U.S.S.R. during the war is out of selfish interests, so the fact that they're stealing information from the U.S. at the same time is totally justified.
The FBI is also watching a man named Haakon Chevalier—a known Communist, and a friend of Oppenheimer's. When Oppenheimer goes to his friend's house for dinner in 1941, the FBI decides to open a file for him, too…just in case.
Meanwhile, the war continues to get worse. Germany and Japan decide to team up, expanding the arena to Asia as well as Europe. The U.S. cuts off oil exports to Japan in order to strangle their ability to wage war, but all it does is make them even more determined (oh, and mad at the U.S.—whoops).
The Uranium Committee is still barely creeping along. Frustrated by their lack of initiative/results, U-Committee member Ernest Lawrence urges them to involve Oppenheimer, his Berkeley colleague.
In the fall of 1941, when Oppenheimer attends his first U-Committee meeting, he's instantly inspired. Now he knows how he can use his genius abilities to aid the war effort: by devising and building an atomic bomb.
December 7, 1941: Pearl Harbor is attacked by the Japanese. The bombs destroy over a dozen warships, hundreds of planes, and kill thousands of American soldiers. It's a devastating blow.
The U.S. immediately declares war on Japan—and Hitler declares war on the U.S.
It's on: the Allied Powers (United States, Great Britain, and the Soviet Union) versus the Axis Powers (Germany, Japan, and Italy).
Pearl Harbor also serves as a catalyst for Oppenheimer: Realizing the urgency needed in regards to building an atomic bomb, he gives up on his discussion groups and starts getting together a team. His first recruit is Robert Serber, a former student he convinces to move into the apartment over his garage as his assistant, a.k.a. "Coordinator of Rapid Rupture."