The most direct political attack on the Unity of Science movement began early, in 1939, as its leading members commenced the Fifth International Congress for the Unity of Science at Harvard University. Those convening were happy to be reunited, but the occasion was not joyous. Most had probably heard the news while traveling to the United States or to Cambridge: Hitler had invaded Poland, and the situation looked grim. On the eve of the Congress, Sunday, September 3, they gathered around a radio to hear President Roosevelt's weekly radio address and learned that Hitler had not backed down from England's and France's ultimatums demanding Nazi withdrawal from Poland (Neurath 1946, 78).

This gloom affected the movement for several specific reasons. Its conferences, publications, and publicity (Time sent a reporter to this conference (“Unity at Cambridge” 1939)) were intended not only to inject empiricist reforms into philosophy, to eliminate spurious metaphysical thinking, and to popularize unified science. These reforms themselves would potentially improve communication and understanding among nations and thus facilitate international cooperation in social and economic planning. But this enlightenment agenda seemed to fall on deaf ears, for war was breaking out and the world was growing darker.

Another more proximate force against the movement appeared at this conference, as well. Horace Kallen was a New York philosopher who had embraced Neurath and logical empiricism both intellectually and socially. At the conference, however, he sounded his alarm that the movement was “totalitarian.