The deterioration of US diplomacy created serious additional pressure on the government to prepare for war. Some American scientists knew the work of their Japanese counterparts, Yoshio Nishina and Hideki Yukawa. Few regarded Japanese nuclear physics as a threat to the West. But the Roosevelt administration did worry about the Japanese military challenge in Asia. As early as 1937, when the Japanese had created a pretext for invading China, Roosevelt had urged his fellow citizens to ‘quarantine’ aggressors, to isolate them diplomatically so as to prevent a contagion of aggression— disease being a prized American metaphor for inimical ideologies borne by war. There followed a series of tit-for-tat measures undertaken by both governments: Japanese aggression followed by a partial US embargo on scrap metal (or, in the Japanese calculus, the encirclement of Japan by Western imperialists in Asia precipitating a Japanese effort to break the confining ring); Japanese designs on the resource-rich European colonies in Southeast Asia responded to by a full US embargo of scrap, along with oil; efforts by Japan’s civilian government to achieve a modus vivendi with the United States on the basis of Japan relinquishing any claim in Southeast Asia in return for US acceptance of its position in China and a resumption of metal and oil shipments—and so forth. Roosevelt’s secretary of state, Cordell Hull, resisted what he considered the appeasement of the Japanese, and by the time Compton and the others had resolved to move forward with the bomb in September 1941, relations between the nations neared the breaking point. The Japanese civilian government fell in November, supplanted by a military-dominated regime headed by General Hideki Tojo.
The new government resolved to strike a blow at the US Pacific Fleet, based at Pearl Harbor, Hawaii. Tojo reasoned that the blow would demoralize the Americans and cripple their capacity to respond to further planned attacks in Asia and the Pacific. On 7 December 1941 scores of Japanese planes—fighters, dive-bombers, and torpedo planes—surprised the Americans Just after dawn. In two precisely flown waves they did grievous harm: 2,400 Americans killed, nearly 1,200 more wounded, 8 battleships and 3 each of cruisers and destroyers sunk or damaged, 300 aircraft destroyed or badly damaged. The next day, a somber Roosevelt requested from Congress, and got, a declaration of war against Japan. ‘Always we will remember the character of the onslaught against us,’ he said. Americans did remember, and they demanded retribution.12
The Japanese attack brought the United States into war, and war became the context for the American quest for an atomic bomb and lent urgency to the quest. Some American strategists imagined from the first that the bomb would be used against the Japanese, because of Pearl Harbor and subsequent Japanese maltreatment of American prisoners of war, and because they feared that, if a bomb dropped on an enemy was a dud, the Germans, not the Japanese, might profitably dissect it for their own purposes. Generally speaking, however, the government and especially the scientists involved with S-1 had Germany in mind as the target for their weapon. They thought German aggression even more brazen and threatening than Japanese. They also believed that Nazism was more heinous than Japanese militarism. And they were very much afraid that German scientists were ahead of them, or at least even with them, in the race for the bomb. Fear of German progress toward a working bomb moved Leo Szilard to urge secrecy on the nuclear physics community early in 1939 and to seek help from Einstein to gain the President’s notice. The Szilard/Einstein letter that resulted drew pointed attention to the German bomb threat, and Ernest Lawrence and Arthur Compton both made prominent mention of a possible German bomb when they gave their reasons for pushing ahead with the US program. Every German move that portended work on nuclear weapons—the ban on uranium exports, the capture of Norway’s heavy-water plant at Vemork, the information from Paul Rosbaud that German physicists were taking a bomb project seriously—brought anxiety to the American scientific community and a renewed determination to forge ahead with its own work on the bomb.13