Japan advanced to southern French Indochina during July 1941 (the 16th year of the Showa Era). The United States, which was increasingly cautious about Japan’s southward expansion policy, imposed an oil embargo on Japan. Why did Japan advance to southern French Indochina? Could Japan predict that the southward expansion policy would invite U.S. sanctions and lead to the Pacific War?

Advancing into French Indochina

 There were several reasons for the decision by Japan to advance to southern French Indochina. They included the prolongation of the Sino-Japanese War, Japan’s thirst for natural resources in Southeast Asia, economic sanctions by the United States and the start of the Russo-German War.

 These factors were complicated and intertwined. But Japan was buffeted by a rapidly changing international situation. It declared war on Britain and the United States about four months after its advance to southern French Indochina.

 It was Germany’s blitzkrieg, or lightning war, in Europe from May 1940 that triggered Japan’s southward expansion policy. As a result of this lightening war, Britain, France and the Netherlands, which held colonies in Southeast Asia, were facing a crisis. It was at this point that debate in Japan intensified on whether to advance to these colonies. The colonies were rich in such natural resources as oil, rubber and tin. Japan thought that if it obtained these natural resources, it would be able to expand its self-­supporting areas and no longer have to rely on Britain and the United States.

 The first step of the southern expansion policy was the advancement to northern French Indochina. At the time, Britain, France, the Soviet Union, the United States and other countries continued to give military support to Chiang Kai-shek’s administration in China that had shifted its capital to Chongqing. As Japan judged that such logistical supplies supported China’s ability to resist Japan, it tried to alter the course of the Sino-Japanese War by cutting the main supply route to Chiang Kai-shek’s forces that ran through French Indochina, now Vietnam, Laos and Cambodia.

 In June 1940, after the fall of Paris, France at last decided to shut down the China-bound supply route. Foreign Minister Yosuke Matsuoka and French Ambassador to Japan Charles Arsene-Henry reached an agreement at the end of August to close down the route. Japan advanced into northern French Indochina on September 23, 1940.

 But the Japanese troops hinted at the use of military force and struck a coercive posture toward local negotiators, which resulted in military clashes. Kyoji Tominaga, chief of the First Bureau (Operations) of the Army General Staff, actively utilized military force in the southern expansion policy and his tendency to exceed his authority invited trouble. After returning to Japan from the mission, he was dismissed from his post.

 The Sino-Japanese War began to look like it would be a drawn out affair. Shinichi Tanaka, chief of the First Bureau of the Army General Staff, later wrote in his diary that in the autumn of 1940, there was no possibility of solving the Sino-Japanese War as “a local conflict” anymore.

 Japan envisaged containing the Soviet Union and halting assistance from Britain and the United States to China by applying the pressure of the Tripartite Pact, by waiting for an opportunity to advance on the Asian colonies of Britain and France and thus by securing natural resources to prepare for a long war with China. Beyond that, Japan pictured taking control of French Indochina.

 The Imperial Japanese Navy supported the Tripartite Pact on condition that the budget and resources would be distributed preferentially to the Navy. This was also made clear in the diary of Matome Ugaki, chief of the Operations Bureau of the Naval General Staff. But before the Tripartite Pact was concluded, the United States announced an embargo on the export of scrap iron to Japan. The Navy’s sense of crisis over the U.S. stance in strengthening the embargoes on exports to Japan resulted in an active southward expansion policy.

 The Japanese Foreign Ministry pressed the Dutch East Indies (now Indonesia) authorities during negotiations to increase the supply of important materials. But the Netherlands’ government was in exile in London and the negotiations with it floundered. As Japan became an ally of Germany, an enemy of the Netherlands, after the conclusion of the Tripartite Pact in September 1940, it was hardly conceivable that the Netherlands would cooperate with Japan. In May 1941, there was no prospect for a completion of the negotiations.

 The deadlock in the negotiations over the Dutch East Indies invigorated the Japanese military’s policy to advance to southern French Indochina. The Japanese troops calculated that if they set up their military bases in southern French Indochina and put military pressure on these areas, the Dutch East Indies would then supply oil to Japan.

 In addition, if Britain were defeated and Japan had a chance to advance to British Malaya and Singapore, Japan would be unable to cope because of the limited range of the bombers from the Japanese bases established in Hainan Island and northern French Indochina. Japanese leaders thought that the United States would allow Japan to advance to southern French Indochina. Japan’s leaders, therefore, wanted advance bases in southern French Indochina.