Aghast at the use of nuclear weapons in August 1945 to obliterate the cities of Hiroshima and Nagasaki, Einstein threw himself into efforts to prevent worldwide nuclear annihilation. In September, responding to a letter from Robert Hutchins, Chancellor of the University of Chicago, about nuclear weapons, Einstein contended that, “as long as nations demand unrestricted sovereignty, we shall undoubtedly be faced with still bigger wars, fought with bigger and technologically more advanced weapons.”