The beginning of World War II changed everything for America's physicists. Suddenly, theoreticians who had spent their professional lives lost in abstract thought were called upon to put their theories to work creating practical applications. Suddenly, physics was relevant.

Historian Daniel Kevles has called World War II "the physicists' war," and with good reason. The wartime physics effort represented the largest and most expensive physics program in history, and physicists emerged from the war having created two of the century's greatest technological triumphs: radar and the atomic bomb. Both have been credited with winning the war for the Allied forces.

One of the two great physics projects of World War II, the development of radar, was already well under way by 1941. Some of the country's greatest physicists were gathered at the Massachusetts Institute of Technology's "Radiation Laboratory"–or "Rad Lab"–racing to create a working radar system. They assumed that this would be the great physics project of the war. They were wrong.

Three years before, the German physicists Otto Hahn and Fritz Strassman had made a startling discovery that would gravely impact the course of human events. They had no idea of this at the time, of course. All they knew was that when you bombarded a uranium nucleus with neutrons, you could split the nucleus and a release a huge amount of energy. Hahn and Strassman had discovered nuclear fission.

As it turned out, when one neutron struck the uranium nucleus, two or three new neutrons would be released, which meant, under the right circumstances, a chain reaction could be created with more and more neutrons. Thus more and more energy would be released at every step. The fissioning of a single uranium atom would release 200 million volts of energy.

The discovery had explosive possibilities if nuclear fission could be harnessed to create a powerful bomb, more powerful than any weapon mankind had seen before. The European physicists in the United States, many of whom had fled Hitler's Nazi regime, immediately realized the possibilities of nuclear fission and feared what would happen if Hitler's scientists realized them as well.

Two of these European émigrés, Edward Teller and Leo Szilard, believed it was essential to convince the American government of the importance of nuclear fission along with the dangers of a German nuclear program designed to study and implement it. So they turned to a man they knew had enough intelligence to understand the problem and enough clout to communicate it to the government: Albert Einstein. Einstein was a pacifist, but he was also devoutly anti-Nazi, and he was eventually convinced that the government must be warned. He drafted a now famous letter to President Franklin D. Roosevelt, explaining the scientists' concerns.

FDR was soon convinced, and he created the Advisory Committee on Uranium, which began to assemble a "pile": a stack of uranium bricks that could be used to create a nuclear chain reaction. But the Advisory Committee was moving too slow, and it was eventually dissolved and replaced by the National Defense Research Council (NDRC). In September of 1942, the NDRC handed the nuclear development project over to the army and appointed Brigadier General Leslie Groves to take control. Groves called his new project by an intentionally misleading code- name; the nuclear project would henceforth be known as the Manhattan Engineer District or, more commonly, the Manhattan Project.

Oppenheimer knew none of this; the earliest top-secret discussions of nuclear power were restricted to an elite few. Oppenheimer was aware of the recent discoveries in nuclear physics and had in fact, since 1932, been doing his own research in the area. But he was unaware of the controversy, fear, and urgency that surrounded the subject. All Oppenheimer knew was that he wanted desperately to get involved in the war effort in some capacity, and, in 1941, he got his chance.

In 1941, Ernest Lawrence invited Oppenheimer to a secret meeting of scientists, one of the earliest meetings to discuss how one would go about building a nuclear bomb. Oppenheimer almost wasn't invited – the government knew of his radical ties and was nonplussed at the idea of exposing a potential communist to top government secrets. But Lawrence was determined to include Oppenheimer, so suggested that his friend sever his few remaining ties to radical organizations. Oppenheimer did and consequently was invited into the inner circle. He was soon placed in charge of the theoretical physics group charged with designing the bomb itself. And this was only the beginning.