The outbreak of the First World War brought Einstein's pacifist sympathies into public view. Ninety-three leading German intellectuals, including physicists such as Planck, signed a manifesto defending Germany's war conduct. Einstein and three others signed an antiwar counter-manifesto. He helped to form a nonpartisan coalition that fought for a just peace and for a supranational organization to prevent future wars. As a Swiss citizen Einstein could feel free to spend his time on theoretical physics, but he kept looking for ways to reconcile the opposing sides. "My pacifism is an instinctive feeling," he said, "a feeling that possesses me because the murder of men is disgusting. My attitude is not derived from any intellectual theory but is based on my deepest antipathy to every kind of cruelty and hatred."

With his scientific fame Einstein could act as unofficial spokesman for the Weimar Republic, and he protested the continued hostility of Germany's former enemies. In 1921 he refused to attend the third Solvay Congress in Belgium, since all other German scientists were excluded from it. In 1922 he joined a newly created Committee on Intellectual Cooperation set up under the League of Nations. The next year he resigned, distressed by the League's impotence when confronted with France's occupation of the German Ruhr. But he soon returned to the committee. As a leading member of the German League for Human Rights, he worked hard for better relations with France. He also made numerous gestures against militarism.

Einstein attracted attention to a number of causes, such as the release of political prisoners and the defense of democracy against the spread of fascism. He spoke in public, made statements to the press, signed petitions. In 1924 he defended the radical Bauhaus School of Architecture; in 1927 he signed a protest against Italian fascism; in 1929 he appealed for the commutation of death sentences given to Arab rioters in British Palestine.