The fundamental disagreement between Bohr and Einstein took place over the issue of indeterminacy. Bohr's principle of complementarity acknowledged that certain atomic behavior can never be exactly measured. For example, one of the main scientific issues was whether or not light and matter were waves or particles. Bohr argued that Instead of using only one base of knowledge, a scientist should recognize that the two complement each other and can lead to a more complete understanding than either in itself. Through the exhaustion of all possibilities, probability could be determined, though behavior could never be predicted with 100 percent accuracy.

While Einstein may have been forced to recognize certain dualities, he would never acknowledge that this understanding was as complete as possible. His famous statement that God doesn't play dice with the universe expresses his refusal to accept that chance plays a role in behavior. He believed that all action could ultimately be understood and predicted if the laws were properly determined. Our inability to predict merely reflected the fact that we had yet to achieve the appropriate understanding and find the right methods. He saw Bohr's complementarity as an incomplete picture of reality and believed that scientists should not rest without completing that picture. Thus, no matter how hard Bohr attempted to show that one could never determine the velocity of an electron without disturbing its position, Einstein would not be satisfied that no theoretical way existed.

Bohr felt he owed his every opportunity to his country, and the University of Copenhagen in particular. When offered a permanent position in Manchester with Rutherford, Bohr preferred to stay at Copenhagen, which was struggling to get its own institute off the ground, because he felt a duty to those who had made his career possible.

At the same time, Denmark adored and was immensely proud of Bohr. He was awarded the highest honors of his country and considered a national icon. As a public figure, he used this status for good and served as an effective symbolic leader. When the Nazis invaded, Bohr stayed as long as was possible, even until the very day when he knew he was to be arrested. He recognized that his departure would weaken the morale of the nation, and particularly its Jewish citizens, as his mother had been from one of Denmark's prominent Jewish families. When he did finally escape, he made sure to do everything he could to help Denmark Jews to safety, working with the Danish resistance despite threats to his own life.

Bohr's always wanted to see nuclear energy used for productive rather than destructive purposes. If scientists could learn to harness it properly, nuclear energy could serve as an invaluable resource. Bohr believed that the way to use nuclear energy as a resource was by fostering cooperation among nations. When politicians wouldn't listen to his advice, Bohr attempted to take the first step through the scientific community. He played pivotal roles in organizations such as Atoms for Peace, which awarded him its first annual award.

Bohr was also one of the first to recognize the dangers that atomic power could have. He foresaw the development of the arms race even as scientists were taking only the first steps toward creating the bomb. Bohr made endless efforts to campaign for international cooperation. He believed that openness would prevent abuses of atomic power. During the war, he even met with Churchill and Roosevelt, but ultimately he could not change this course of history.