Between July 1940 and the Summer of 1941, the war in China continued. The military situation in China was characterized by a transition into a brutal war of attrition against Japanese occupation, alongside a significant internal breakdown in the alliance between Chinese Communist and Nationalist forces. Among major actions was the “Hundred Regiments Offensive” (Aug 1940 to Jan 1941). It was the largest Communist-led offensive of the war, involving roughly 400,000 troops. It targeted Japanese-held infrastructure, specifically railroads and mines, in northern China to disrupt supply lines. In retaliation, the Japanese initiated the “Three Alls” policy: kill all, burn all, loot all. It was a scorched-earth policy, leading to widespread massacres and the destruction of thousands of villages.

Meanwhile to the south, the Nationalist Army enjoyed some victories and endured some losses. It was a clear implementation of the “war of attrition” policy against the Japanese.

During this period the U.S. continued to provide supplies via the Burma Road and began formalizing military aid through the Lend-Lease program, which included the procurement of P-40 aircraft for the American Volunteer Group, known as the “Flying Tigers.”

The Turning Point

In July 1941 Japan moved into Southern Indochina. This was the “bridge too far.” By mid-1941, Japan’s strategic position had become increasingly precarious. The war in China showed no sign of resolution, Japan’s economy was under strain, and dependence on foreign, especially American, oil had become acute. The occupation of southern Indochina in July 1941 represented a decisive escalation driven by both necessity and ambition, as Japanese leaders concluded that time was working against them.

The immediate rationale lay in Japan’s southern expansion strategy (Nanshin-ron), which had become the strategic plan after the defeat at Nomonhan at the hands of the Soviet Union. Southern Indochina offered strategic airfields and naval bases particularly around Saigon and Cam Ranh Bay. This placed Japan within striking distance of British Malaya, Singapore, and the Dutch East Indies, the latter being Japan’s most coveted objective due to its vast oil reserves. Control of southern Indochina would serve as a springboard for future operations, not merely a continuation of the China war.

Japan also viewed the move as defensive and deterrent. Japanese planners feared that continued U.S. and British pressure would eventually choke off vital imports. Occupying southern Indochina was intended to secure strategic depth, signal resolve to Western powers, and strengthen Japan’s negotiating position.  Coercion had proved successful before and so Japan stayed with what worked.

Japanese leaders still hoped to avoid war with the United States but the nationalist and military believed that a show of strength would compel Washington to accept Japan’s dominance in East Asia or at least negotiate a settlement recognizing Japan’s “special position.” As with earlier expansions, Tokyo framed the occupation as temporary and stabilizing, carried out with Vichy French acquiescence rather than outright conquest.

Unlike the occupation of northern Indochina in 1940, which could be justified as cutting Chinese supply lines, the move south had no plausible defensive rationale. It directly threatened Western colonial holdings and, crucially, placed Japanese forces astride the sea lanes connecting the United States, Britain, and Southeast Asia. For American policymakers, southern Indochina marked the point at which Japanese intentions could no longer be interpreted as limited or negotiable. Their goal of regional domination, far beyond trade, was unmistakable. All signs were that Japan was preparing for an offensive war – which was exactly the Nanshin-ron strategy.

The U.S. Internal Debate

The Japanese move triggered an intense but brief debate within the Roosevelt administration. The debate was brief because all the arguments had already been raised during earlier crises. Within the State Department, Cordell Hull concluded that Japan had crossed a qualitative threshold. While Hull had previously opposed measures such as an oil embargo on the basis that it might force Japan into a corner, he now accepted that failure to respond decisively would invite further expansion. Southern Indochina confirmed Hornbeck’s and others’ arguments that incremental pressure and diplomacy were never going to constrain Japan.

At the same time, Treasury Secretary Morgenthau and others argued that the United States had been subsidizing Japanese aggression through continued trade, especially petroleum exports. Morgenthau pressed for immediate financial measures that would cut off Japan’s access to dollars and strategic materials. President Roosevelt, who had long sought to balance deterrence with delay, now sided with the more forceful camp.  It was not because he sought war, but because he believed that credibility and long-term security required drawing a firm line. Southern Indochina convinced Roosevelt that ambiguity no longer served U.S. interests.

U.S. Actions and Their Consequences

In response, the United States took a series of actions that fundamentally altered the strategic environment. Two coordinated actions were put in place. In July 1941 the U.S. froze all Japanese financial assets in the United States. The funds were available when connected to a valid and approved export license. The 1940 Exports Control Act already required an export license for oil and oil products, but now companies and purchasing agents had to navigate the dual administrative processes. At no point did the U.S. formally announce an oil embargo, but a de facto embargo was enacted by these two administrative processes that could “slow roll” any license applications. These two actions effectively prevented Japan from purchasing American goods, including oil, as approvals became trapped in the bureaucracy of the two separate processes. That being said, Japan’s petroleum supply from the U.S. was effectively cut off. Given Japan’s heavy dependence on American oil, they viewed this as an existential threat.

Britain and the Netherlands soon imposed similar freezes, closing off alternative sources in Southeast Asia. Japan now faced the prospect of economic strangulation within a year if no resolution was reached. 

These measures were intended to force Japan back to the negotiating table. American leaders hoped that the severity of the response would compel Japan to halt further expansion and reconsider its position in China. Instead, the effect was the opposite: Japanese leaders increasingly concluded that only force could secure the resources Japan needed to survive and continue their military expansion.

The occupation of southern Indochina was Japan’s final major expansion before U.S. involvement in the Asia Pacific War. From their point of view it was driven by strategic desperation, resource insecurity, and overconfidence in coercive diplomacy. For the United States, it marked the moment when gradualism gave way to decisive economic action. The resulting asset freeze and effective oil cutoff were not intended as steps toward war, but they made war increasingly likely. 

Japanese leaders were nationalistic and supported the military. Their analysis of history was that it was only with military power and control that Japan’s future against western powers could be secured. And so for them they saw that peaceful options had run out. The irony is that for the previous four years, Japan had never taken a peaceful option but had always exercised the military option – and always via surprise attack and mobilization: Mukden, Nomonhan, and soon enough, Pearl Harbor.

The Japanese move into Southern Indochina was the irreversible hinge between diplomacy and conflict. It was the moment when both sides believed they were acting defensively, yet set in motion the final march toward Pearl Harbor. 

Image credit: various photographs from Naval Aviation Museum, National World War II Museum, and US Navy Archive

By June 1939 Japan was deeply entrenched in China with a military stalemate, the Chinese willing to fight a war of attrition, and there was no political settlement in sight. Japan had just suffered a major defeat at Nomonhan, though this was not fully appreciated in Washington at the time, and elements of Japanese leadership were increasingly suspicious of Western intentions. That same month the U.S. formally notified Japan it would terminate the 1911 Treaty of Commerce and Navigation, effective January 1940. This did not impose sanctions immediately but freed the U.S. legally to restrict trade later. The purpose of the actions was to signal displeasure with Japan’s conduct in China, create leverage without escalation, and preserve diplomatic ambiguity. It was not well received in Japan as they understood that beginning in January 1940 there would be no treaty in place that could limit trade restrictions. At the same time the State Department issued nonbinding requests to U.S. firms not to sell aircraft, aviation fuel or strategic materials to Japan. This reflected Hull’s belief that economic pressure should precede coercion, and coercion should precede war. This would come to be known as the “moral embargoes” and marked the first move from moral pressure towards economic leverage

French Indochina

A key reason China could continue fighting was that foreign supply routes remained open, especially the Burma Road and rail and port access through French Indochina, particularly via Haiphong into Yunnan Province.From Tokyo’s perspective, cutting these supply lines became essential. Japan was presented a strategic opportunity with the collapse of France to German forces in June 1940. As a consequence the Vichy regime replaced the French government. This left French colonial authorities isolated, under-resourced, and politically uncertain. To Japanese planners, Indochina now looked militarily weak and diplomatically unsupported and unlikely to receive British or American military support in the short term. To the Army, Navy, and politicians this created a low-risk window of opportunity.

Japan initially pursued its objectives through coercive diplomacy, not outright invasion. Japan demanded that Vichy France close supply routes to China, permit the Japanese to station inspectors at transportation junctions (and later military forces), and provide unfettered access to airfields in the North. Vichy France wanted to preserve sovereignty but lacked the military means to resist and so hoped that accommodation would prevent full occupation. Negotiations dragged on while Japan prepared militarily, a familiar pattern since Mukden.

Despite French promises, Japan believed that supplies were still leaking into China, French officials were unreliable, and only physical control could guarantee closure of routes. The Japanese Army argued that diplomatic assurances were meaningless without troops on the ground, reflecting the Army’s broader pattern of fait accompli strategy.

In September 1940, Japanese forces crossed into northern Indochina and occupied key airfields and rail lines. They clashed briefly with French colonial troops but soon enough all resistance collapsed. Shortly afterward a formal agreement legalized the Japanese presence while French administration remained nominally in place.  Japan was fine with leaving administration to the French because they had gained what they wanted: control of transport corridors and air bases from which to control supplies into China.

Japan limited itself initially to northern Indochina because the stated goal was cutting China’s supply lines and it allowed Japan to test Western reactions in an incremental way as it avoided directly threatening oil supplies.

To the international community, Japan deliberately framed this as a “temporary” measure, a defensive necessity and not an annexation. But while China was the immediate justification, larger strategic calculations were at work. After the defeat at Nomonhan, the Northern expansion against the USSR lost credibility and attention shifted south toward Southeast Asia. Indochina offered a stepping stone toward the Dutch East Indies with airfields within reach of British Malaya. Japan also believed a decisive move would demonstrate resolve to the point the U.S. and Britain would protest but would not fight or take any decisive action. Internally it provided prestige to the military and the government and was promoted at home. Japan calculated that in the short term Britain was fully engaged in a battle for its national life (the “Battle of Britain”) while the U.S. was still divided and formally neutral.

The U.S. response was as Japan expected: a strong diplomatic protest, limited export controls, but no other embargo action. All in all, this reinforced Japanese beliefs that incremental expansion would work.

Reaction with the U.S. Government

The U.S. response to Japan’s occupation of northern Indochina exposed deep internal divisions within the Roosevelt administration over how far and how fast to confront Japan. At the center of the debate was a shared recognition that Japan had crossed an important threshold, but no consensus on whether that threshold justified decisive economic retaliation or continued diplomatic caution.

Within the State Department, Secretary of State Cordell Hull favored a measured, incremental response. Hull believed Japan’s move was aggressive but still reversible and that premature, sweeping sanctions, especially on oil, risked provoking a war the United States was not yet prepared to fight. State Department officials continued to emphasize negotiation, the preservation of legal and moral principles (such as the Open Door in China), and the use of graduated economic pressure to influence Japanese decision-making. Hull and his advisers, especially Ambassador Grew, held out hope that divisions within Japan, particularly between civilian moderates and the military, could still be exploited diplomatically.

By contrast, Treasury Secretary Henry Morgenthau Jr., supported by some in the Interior and Navy Departments, argued that Japan’s occupation demonstrated that incremental pressure had failed; it had been applied for almost three years. This faction favored stronger economic sanctions, including tighter export controls and financial restrictions, to signal that further expansion would carry unacceptable costs. Morgenthau was particularly concerned that continued U.S. trade, especially in oil, gasoline, aviation fuel,  and scrap metal, was materially enabling Japanese aggression. President Franklin Roosevelt ultimately sided, for the moment, with Hull’s caution: the U.S. imposed new export controls and intensified diplomatic protests but stopped short of an oil embargo. The compromise reflected a broader strategic judgment that time was needed to strengthen U.S. defenses while keeping open the possibility, however slim, of restraining Japan without war.

Export Controls Act 

In July 1940 Congress passed the Export Controls Act in response to Japan’s move into northern French Indochina. At the core of the legislation was that the State Department gained authority to license or deny exports. Immediately restrictions were placed on aviation gasoline, high quality scrap iron and machine tools. The licensing process gave the State Department the ability to “approve” the request and then slow march the license through the administrative process where decisions were deliberately incremental and often reversed

The Departments of Treasury and Interior wanted stronger measures and more excluded items. Despite pressure from Treasury and some Navy officials, the administration deliberately excluded oil from the provisions of the 1940 Act. Petroleum exports continued, ordinary commercial goods remained largely unaffected, and financial transactions were not yet frozen. All this reflected Secretary Hull’s view that an oil embargo would be indistinguishable from a declaration of economic war. This marked a clear escalation in U.S. policy, but one carefully calibrated to apply pressure without forcing an immediate showdown.

Image credit: various photographs from Naval Aviation Museum, National World War II Museum, and US Navy Archive

In the summer of 1939 Japan was increasingly bogged down in combat with China. As you can see in the map below, Japan controlled not only Manchukuo (Manchuria) but swaths of Inner Mongolia and China. Importantly, Japan also controlled almost all major seaports facing the East and South China Sea. Manchukuo presented its own control issues, largely centered around rampant banditry, but all the other areas required the Kwantung Army (Imperial Japanese Army on the mainland) to provide occupation and policing forces in areas they had conquered. At the same time the Kwantung Army continued offensive operations in Northern China.

Notice that Japan also had an extensive border with the Soviet Union as well as a history of conflict with czarist Russia that extended some 60 years back in time including the 1904-1905 Russo-Japanese War which removed Russian presence in Manchuria and Korea. Russia lost the warm water ports in Darien and Port Arthur, access to the resources of Manchuria, and was often required to transport goods and resources via the Trans-Siberian Railway rather than via ship. In addition Russia was required to cede the lower half of Shanklin Island (just north of Japan). All of this fit into Japan’s “strategic buffer” concept keeping China and Russia/Soviet Union at “arm’s length” from the home islands.

The Nomonhan Incident

Also known as the Battles of Khalkhin Gol, the Nomonhan Incident was a major but undeclared border war fought between Imperial Japan and the Soviet Union’s Mongolian forces from May to September 1939. It occurred along the poorly defined frontier between Japanese-controlled Manchukuo and Soviet-aligned Mongolia. Though officially termed an “incident” by Japan to avoid acknowledging a formal war, the conflict involved tens of thousands of troops, armor, artillery, and aircraft, making it the largest land battle Japan fought before the Pacific War. The fighting ended in a decisive Soviet victory, delivering a shock to the Japanese military and reshaping Japan’s strategic direction on the eve of World War II.

The Background

At the root of the conflict was a disputed, ambiguous border. Japan claimed the boundary lay along the Khalkhin Gol (Halha River), while the Soviets and Mongolians insisted it lay several miles east. The area was remote, sparsely populated, and economically marginal but symbolically important as a test of sovereignty and power. It was also extremely remote from the Imperial Japanese Army (IJA) Staff and Ministry in Tokyo. All of this was simply ingredients of a toxic stew only awaiting the flame to start a boil. And the Kwantung Army was only too happy to ignite the flame.

The Kwantung Army had a well earned reputation for acting autonomously from Tokyo, initiating aggressive actions to force what it believed were the necessary political outcomes, and viewing itself as the vanguard of Japan’s continental destiny. This attitude led to the Mukden Incident (1931), the Marco Polo Bridge Incident (1937), and the quagmire of the Second Sino-Japanese War. By the summer of 1939 the IJA was already stretched thin, had terrible logistics support, and was lacking in combined arms capability – meaning tanks, artillery and ground support from aviation assets. Financially, Japan was stretched thin in trying to keep the IJA funded at the same time funding the Navy’s capital intensive shipbuilding efforts.

Nonetheless, by 1939 the bane of IJA’s existence – an independent, insubordinate junior officer corps believed that controlled escalation against the Soviets could secure Japan’s northern frontier, expand the strategic buffer or even open the door to expansion into resource-rich areas of Siberia. In the view of Japan, the majority of the peoples in Siberia and Mongolia were Asiatic and thus properly best served under the Imperial Protection of the Emperor. They did not bother to ask the people of those areas about their preference.

Northern Expansion Doctrine (Hokushin-ron)

Within Japan’s strategic debates, the Army strongly favored Hokushin-ron, the idea that Japan’s future lay in expanding northward against Russia rather than southward into Southeast Asia. This belief rested on overconfidence from victories in China, an underestimation of Soviet military modernization, and a deep seated anti-communism which was, at this time, being suppressed in the home islands. Nomonhan emerged from a belief that the Red Army could be tested, pressured, and defeated through limited engagement. It was less a grand strategy and more a tactical escalation.

In part it was also pushing back on the evolving southern expansion doctrine being proposed by the Imperial Japanese Navy (IJN) whose interest lay in the resource-rich Southwestern Pacific, especially the oil fields of Borneo, Malay and Java.

The local IJA commanders were confident that Japanese infantry superiority would compensate for weaknesses in armor and logistics. In addition, they believed that the Soviets would avoid a major escalation. The Kwantung Army assumed that a sharp, localized victory would strengthen Japan’s hand diplomatically and militarily.

The IJA horribly misread Soviet intentions. The Soviets retained the memory of the humiliating defeat by Japan of czarist Russia. It was a lingering shame that the Soviets would never let happen again. They were more than willing to fight. They also had superior tanks and artillery and a capacity for combined arms operations with their ground forces. Japan also failed to recognize that the Soviets had the same essential “strategic buffer” view as Japan. Mongolia as part of that buffer and the Soviets would respond decisively to any threat in that region.

Combat

In the period May–June 1939, the initial clashes involved cavalry and infantry skirmishes for small areas of land as well as an island in the middle of the border river. Japanese forces crossed into disputed territory, driving out Mongolian units. Early successes reinforced Japanese confidence.

In June, the Soviets appointed General Georgy Zhukov to command. Zhukov rose to prominence with his leadership at Nomonhan. He was a brilliant tactician and field commander later becoming the most prominent and successful Soviet military commander during World War II often credited as the key strategist behind major Eastern Front victories against Nazi Germany. He rose to the position as Chief of the General Staff and a Marshal of the Soviet Union, leading the defenses of Moscow, Leningrad, Stalingrad and commanding the final assault on Berlin. He was formidable. 

Long story, told short, Zhukov prepared a coordinated counteroffensive rather than piecemeal retaliation, amassing artillery, tanks, and aircraft supported by secure logistics/supply lines. In late August, Zhukov launched a classic double-envelopment, using tanks and mechanized infantry to encircle Japanese forces who lacked effective anti-tank weapons, were undersupplied, and relied on infantry assaults against armor. The result was catastrophic. Entire Japanese formations were destroyed or rendered combat-ineffective. It was a clear battlefield defeat involving some 20,000 Japanese casualties with the loss of experienced officers and elite units. Japan agreed to a ceasefire in September 1939, restoring the status quo but psychologically, the damage was done. It was the first unequivocal defeat suffered by Imperial Japan since the Meiji era.

Unintended Consequences

Nomonhan should have shattered Japanese assumptions – but it did not. Japan continued to believe that Japanese fighting spirit and bushido were enough to triumph and could overcome material inferiority and weak logistics. This would plague them all the way through 1945. They also continued to believe that wars could be tightly controlled and limited.

The defeat decisively undermined the Northern Expansion faction within the Army. After 1939 all plans for war against the Soviets were shelved, a defensive posture was adopted along the Manchurian frontier, and Japan avoided conflict with the Soviets for the remainder of WWII. This shift paved the way for Nanshin-ron (southern expansion), pushing Japan toward Southeast Asia and the Dutch East Indies’ oil. It was the path to confrontation with Western colonial powers. In this sense, Nomonhan helped set Japan on the road toward Pearl Harbor.

Perhaps the most profound unintended consequence was for the Soviet Union. Stalin gained confidence that Japan would not attack in the east and soon enough the two nations signed the Soviet–Japanese Neutrality Pact (1941) allowing Soviet divisions to later be transferred west to defend Moscow against Germany. The Nomonhan incident influenced the outcome of the European war, not just Asian geopolitics.

The Nomonhan Incident stands as a warning ignored about the limits of aggression driven by ideology rather than sound political and military strategy and tactics. It revealed the dangers Clausewitz warned of: wars initiated for political symbolism without a clear path to decisive victory would ultimately end in disaster for those who initiated the action.

Nomonhan was not merely a border clash. It was Japan’s strategic crossroads. Historians have come to now recognize Nomonhan as one of the most consequential “incidents” of the twentieth century – even though it largely remains unknown in the West.

Image credit: various photographs from Naval Aviation Museum, National World War II Museum, and US Navy Archives. | Pacific area map of 1939 courtesy of MapWorks via Wikipedia Commons.