The sole obstacle to Japanese hegemony in South-East Asia was America. On the one hand, it was clear that the United States had scant appetite for war, in Asia or anywhere else. On the other, Americans had little desire to see Japan as sole master of China, let alone the whole of East Asia. But those who ran US policy in the Pacific believed they did not need to take up arms to prevent this, because of Japan’s dependence on trade with the United States and hence its vulnerability to economic pressure. Around a third of Japan’s imports came from the United States, including copious quantities of cotton, scrap iron and oil. Her dependence on American heavy machinery and machine tools was greater still. Even if the Americans did not intervene militarily, they had the option to choke the Japanese war machine to death, especially if they cut off oil exports. This was precisely what made it so hard for American diplomats and politicians to foresee the attack on Pearl Harbor. As normally risk-averse people, they could not imagine the Japanese being so rash as to gamble on a very swift victory when the economic odds were stacked so heavily against them. They assumed that the partial sanctions imposed after the Japanese invasion of Indo-China would send a clear enough signal to deter the Japanese. The effect was precisely the opposite.
The path to war in the Pacific was paved with economic sanctions. The Japanese-American Commercial Treaty of 1911 was abrogated in July 1939. By the end of the year Japan (along with other combatants) was affected by Roosevelt’s ‘moral embargo’ on the export of ‘materials essential to airplane manufacture’, which meant in practice aluminium, molybdenum, nickel, tungsten and vanadium. At the same time, the State Department applied pressure on American firms to stop exporting technology to Japan that would facilitate the production of aviation fuel. With the National Defense Act of July 1940 the President was empowered to impose real prohibitions on the exports of strategic commodities and manufactures. By the end of the month, after a protracted wrangle between the State Department and the Treasury, it was agreed to ban the export of high-grade scrap iron and steel, aviation fuel, lubricating oil and the fuel blending agent tetraethyl lead. On September 26 the ban was extended to all scrap; two months later the export of iron and steel themselves became subject to licence. No one knew for sure what the effect of these restrictions would be. Some, like the State Department’s Advisor on Far Eastern Affairs Stanley Hornbeck, said they would hobble the Japanese military; others, like the US ambassador in Tokyo, Joseph Grew, that they would provoke it. Neither view was correct. The sanctions were too late to deter Japan from contemplating war, since the Japanese had been importing and stockpiling American raw materials since the outbreak of war in China. Only one economic sanction was regarded in Tokyo as a casus belli and that was an embargo on oil. That came in July 1941, along with a freeze on all Japanese assets in the United States – a response to the Japanese occupation of southern Indo-China. From this point, war in the Pacific was more or less inevitable.