In his early days as professor of theoretical physics at Cornell, Feynman gave a number of talks and lectures on the atom bomb. The present manuscript was surely written very soon after arriving at Cornell, as we do not yet see some of the more ethical dimensions of his papers on the topic that would follow later in 1946-47, where he starts to turn his focus more directly on the peace-time applications of atomic power. He makes mention of the Smythe Report, published by the Princeton University Press in August 1945, just a few days after the bombings of Hiroshima and Nagasaki. 

Feynman's article published in the International Control of Atomic Energy Report, co-written with R. F. Bacher, Feynman's colleague at Cornell (and former head of the Bomb Physics Division at Los Alamos Laboratory), is entitled "Introduction to Atomic Energy." It is interesting to note that Richard Tolman, who sent Feynman the pamphlet along with the letter, also worked on the Manhattan Project, and went on to teach, like Feynman did, at Caltech.