ENIAC appeared too late for one war but just in time for another, a Cold War with a different set of military priorities. In this conflict, the demand for firing tables trailed off, replaced by an overriding necessity to perfect the atomic bomb. Even before the bombing of Hiroshima and Nagasaki in August 1945, scientists at the Los Alamos Scientific Laboratory in New Mexico had started working on a hydrogen bomb, a far more powerful and intricate weapon than the uranium- and plutonium-based ones dropped on the Japanese. At the suggestion of John von Neumann, a brilliant mathematician who served as a consultant to both the Moore School and Los Alamos and who had a profound understanding of the potential of computers, the first job given ENIAC had nothing to do with firing tables. (Von Neumann is a central figure in the history of computers and we will discuss his work in the next chapter.) Instead, it was a large and complex calculation of the feasibility of a proposed design for the H-bomb.

The calculation, a mathematical model of an H-bomb explosion, was enormous, with thousands of steps for the program and a million punch cards for the data. Since ENIAC could not store programs or remember more than twenty ten-digit numbers (one set of numbers for each accumulator), the program had to be solved in stages, an exceedingly cumbersome process. The first group of cards was fed into the machine, which punched out intermediate results on other cards, which were resubmitted to ENIAC, and so on, until the mathematical model was calculated. The program was run in November 1945 and the answers, which were available by December, revealed several flaws in the proposed design of the bomb. “The complexity of these problems is so great,” the director of Los Alamos wrote the Moore School in March 1946, “that it would have been impossible to arrive at any solution without the aid of ENIAC…. It is clear that physics as well as other sciences will profit greatly by the development of such machines.”