Einstein wrote a letter to Franklin Roosevelt to warn him about the potential development of "extremely powerful bombs of a new type" by Nazi Germany. He urged the President to take action and support research in uranium fission, which led to the Manhattan Project and the eventual development of the atomic bomb.

Einstein reacted to the bombing of Hiroshima and Nagasaki with horror and regret. He had been a pacifist and had advocated for peaceful uses of atomic energy. The realization that his letter to Roosevelt had indirectly contributed to the development of the atomic bomb led to a deep sense of personal responsibility for the casualties. Later in his life, Einstein became an advocate for nuclear disarmament and a world government.

The world remembers Einstein as a ‘world citizen’ because of his active engagement in global issues, beyond his contributions to science. His advocacy for peace, his warnings against the dangers of nuclear weapons, and his support for civil rights and non-violent resolution of conflicts showed his deep concern for humanity as a whole.

People called Einstein a world citizen due to his involvement in international movements for peace and his efforts to promote global understanding and cooperation. His stance on issues affecting all of humanity, such as the call for a global governing body to prevent war and his efforts to assist refugees, demonstrated his commitment to global citizenship.