How Did Japan Get Involved in World War II?

World War II saw the Allied Powers fight a brutal, globe-spanning conflict against the fascist Axis Powers. Allied Powers Britain, the Soviet Union, and the United States fought Nazi Germany and fascist Italy in Europe and North Africa. In Asia and the Pacific, fellow Allied Power China joined Britain (and Australia and New Zealand) and the United States in fighting the Empire of Japan. How did Japan join Germany and Italy as part of the Axis Powers? Did Japan ever try to link up militarily with its allies? What led Japan to attack the United States Navy at Pearl Harbor, Hawaii on December 7, 1941? Here’s a look at how Japan got involved in World War II.

Japan’s rise as an imperial power can be traced back to the late 1800s when it joined Western powers in setting up spheres of influence in China. This began a rivalry between Japan and Russia for access to Manchuria, the resource-rich region of northeastern China. On February 8, 1904, Japan launched a surprise attack against the Russians at Port Arthur in Manchuria, sparking the Russo-Japanese War. Western observers were surprised at Japan’s ability to defeat the Russians in several engagements, and the United States brokered a peace treaty between the two parties. Russia’s humiliating defeat resulted in the Russian Revolution of 1905 and later influenced the Bolshevik Revolution of 1917.

Having become the dominant power in East Asia, Japan swiftly encroached on Russia’s former sphere of influence in Manchuria. In 1910, after five years of political escalation, Japan formally annexed the Korean peninsula. Allegedly, this was done to maintain order and protect the safety of Japanese residents. Having taken Korea as a de facto colony, Japan became an empire. Thus began the era of imperial Japan, with Japan becoming Asia’s most powerful independent nation. Korea only became independent after World War II, divided into North Korea and South Korea and swiftly falling into the Korean War (1950-53).

Setting the Stage: Second Sino-Japanese War (1931/37-45)

The League’s failure to act, followed by subsequent failure to act in response to Italy’s invasion of Ethiopia in 1935, emboldened Japan to strike again in 1937. The Manchurian Crisis had allowed Japan to create a de facto colony known as the state of Manchuko, and skirmishes between Chinese and Japanese in this region resulted in full-scale war. The Second Sino-Japanese War saw terrible atrocities committed by Japanese troops, especially the infamous Rape of Nanking. After capturing the Chinese city of Nanking, Japanese troops committed mass rape and murder of civilians, resulting in Western outrage and condemnation.

As the Second Sino-Japanese War raged in northern China, the conflict put Japanese troops in close proximity with troops from the Soviet Union – formerly Russia – that were occupying Mongolia. This resulted in skirmishes around disputed border regions beginning in 1938. In the summer of 1939, the Japanese decided to use a full-scale army to dislodge the Soviet Red Army troops and their Mongolian allies from the disputed region in Outer Mongolia. On July 2, the Japanese launched an attack, beginning a prolonged conflict known as the Battles of Khalkhin Gol.

The Japanese were likely expecting a swift victory, similar to their performance in the Russo-Japanese War. Instead, the Red Army struck back in August with a devastating offense designed by General Georgy Zhukov. Although the battle was costly for both sides, it was largely seen as a victory for the Soviet Union by dissuading the Japanese from continuing their push into Mongolia. A cease-fire was agreed to, and the Japanese refocused their attention southward into China. The battle was seen as a precursor to World War II by featuring innovative uses of armor and air power.

September 1940: Japan Joins Axis Powers

The skirmishes between Japan and a surprisingly effective Soviet Union in Mongolia, coupled with Soviet military aid to China, led to Japan seeking a military alliance with another rival of the Soviet Union: Nazi Germany. Seeking to oppose the expansion of communism, which was then an official goal of the Soviet Union, nationalist Japan had an ideological similarity with fascist Germany. Although the 1939 nonaggression pact between Germany and the Soviet Union came as a shock to Japan, American support for both China as well as Britain and France in the growing war in Europe (World War II in Europe was sparked by a German invasion of Poland on September 1, 1939) helped keep Germany and Japan united by a second common rival: the United States.

With Germany and Italy already formally allied, Japan joined the existing alliance on September 27, 1940. This Tripartite Pact, so named for its three members, created the major Axis Powers. The Pact called for the three to support each other in the event that a nation not currently engaged in conflict with any of the trio attacked…referring to the Soviet Union and the United States. Similar to the Triple Entente ahead of World War I, the strategy was to intimidate a rival by having an ally on either side, forcing the rival to divide its forces and defend on two fronts. In the event of war, both the Soviet Union and the United States could be struck from both the west and the east.

Japan and the Soviet Union signed their own nonaggression pact in April 1941. However, in a major shock, Nazi Germany invaded the Soviet Union on June 22, 1941 in Operation Barbarossa. The German invasion was an unwelcome surprise to Japan, sparking debate on what military and political strategies Japan should pursue. One faction argued that the German-Soviet war would reduce pressure in the ongoing Second Sino-Japanese War, as the Soviets could no longer actively assist the Chinese. Another argued that the war was harmful because it meant the US was freer to focus on countering Japan in the Pacific region, with Germany now entirely focused on the Soviet Union.

In the USSR, the German Wehrmacht was enjoying a string of decisive victories as it drove eastward toward Moscow. The Red Army had many forces in the Far East…but was Japan going to invade in support of its German ally? Fortunately for the Soviets, spy Richard Sorge, posing as a German journalist, alerted that the Japanese were not planning on joining the Germans in an attack on the Soviet Union in the near future. This news, delivered in mid-September, allowed the Red Army to send hundreds of thousands of soldiers to counter the German onslaught. In early December, the German offensive was ground to a halt just outside Moscow, preventing the Soviet Union’s defeat (and allowing for its eventual victory).

Autumn 1941: Japan Plans Pacific Offensive

Richard Sorge had been correct – the Japanese had decided not to attack north, into the Soviet Union, but southeast into the Pacific. Tensions with the United States had been growing since the Rape of Nanking, which resulted in US economic sanctions against Japan. In response to an American ban on the purchase of metals and aviation fuel in July 1940, Japan began seizing French Indochina (present-day Vietnam and Cambodia) due to France’s defeat at the hands of Nazi Germany. One year later, Japan began planning an offensive into the South Pacific against British and Dutch territories, which were rich in resources, including oil.

With France and the Netherlands defeated by the Nazis and Britain engaged in fighting in North Africa, only the United States remained in the Pacific to counter the Japanese. Political negotiations were unsuccessful, with Japan refusing to give up any of its seized territory. On July 26, 1941, the United States froze all Japanese assets in the country, meaning Japan could no longer purchase American oil (at the time, the US was an oil exporter). Preparations were begun for an offensive to seize oil in the South Pacific, with the US Navy as the only potential obstacle.

In late November, a massive armada was readied to begin a December offensive across the South Pacific. To prevent the United States from interfering, Japan planned to destroy the US Navy Pacific Fleet, much of which was stationed at Pearl Harbor, Hawaii. Admiral Isokuru Yamamoto planned a major attack, using all six of his aircraft carriers to target the naval base. Early on a Sunday morning, December 7, 1941, hundreds of Japanese planes struck the Hawaiian island of Oahu, sinking several US Navy ships and destroying up to 300 aircraft. In total, over 2,400 Americans were killed in the attack.

Fortunately, despite the combat success of the Japanese attack, the strategic effect was far less than anticipated. None of the three US aircraft carriers stationed at Pearl Harbor were there on the morning of December 7, and more ships than anticipated were saved at Pearl Harbor through heroic efforts. The next day, the US president asked Congress for a declaration of war, which was swiftly given. Thousands of young men volunteered for military service. While Japanese military planners had thought America would reel from the blow of Pearl Harbor, instead, it was prepared to fight. Admiral Yamamoto wrote in his diary, “I fear all we have done is to awaken a sleeping giant and fill him with a terrible resolve.”

December 11: Axis Powers Declare War on America

Japan and the United States were at war by the end of December 8. However, the entirety of World War II did not occur until December 11, when fellow Axis Powers Germany and Italy declared war on the United States. Although a de facto state of war had existed in the North Atlantic between the US Navy and the German Kriegsmarine, it is debatable when and if the US would have joined the war in Europe against Germany and Italy without their December 11 declaration of war against America. Although Germany and Italy likely assumed that the United States would focus primarily on fighting Japan, Allied planners (including the United States) agreed on a Germany First strategy.

Because Germany was close to defeating the Soviet Union, it was agreed that the US would rush military aid to the USSR (which it had been doing since the summer through the Lend-Lease Program). On January 26, 1942, only weeks after Germany and Italy declared war on the United States, the first US troops arrived in Britain. Since 1940, Britain had been subjected to German bombing raids, and any American aid was welcomed. In both Europe and the Pacific, America struck first with air power: Eagle Squadrons of American volunteer pilots had operated in Britain since 1940, and three all-American squadrons were operating by January 1942.

Many Americans may be unaware that Pearl Harbor was not an isolated attack but part of a broad offensive across the South Pacific. On December 7, Japan’s attack on Pearl Harbor had brought it directly into a state of war with the United States. The next day, Japan attacked the British colony of Hong Kong, bringing it directly into a state of war with Britain. On the same day, December 8, both the US and Britain declared war on the Empire of Japan. Japan had itself declared war on both powers simultaneously the day before. Fierce fighting raged in Hong Kong until December 25, when the city’s garrison was forced to surrender.

The largest single defeat in British military history came in Singapore, where the garrison of 85,000 troops surrendered to the Japanese on February 15, 1942 after a bloody siege. Due to the ongoing war in Europe (and the North Atlantic), Britain was unable to effectively resupply its naval base in Singapore. Additionally, Japanese forces were far more experienced in combat due to the ongoing Second Sino-Japanese War, giving them an advantage over British forces in both Hong Kong and Singapore. Britain’s commonwealth allies in the South Pacific, Australia and New Zealand, also entered the war against Japan on December 8, 1941 and were involved in fighting in Singapore. Canadian troops fought in the Battle of Hong Kong, and Canada had declared war on Japan on December 7, fearing a potential attack on its west coast.

The Japanese offensive beginning in December 1941 made the conflict a true world war, roping Britain, Canada, Australia, New Zealand, and the United States into the Asia-Pacific conflict. However, one final event would lock the conflict into a fight until unconditional surrender. Unlike World War I, which ended in an armistice, the brutality of World War II (and lessons from the rise of fascism) made the Allied Powers vow to accept only unconditional surrender from the Axis Powers. Part of the Japanese offensive involved the invasion of the Philippines, a US colony, resulting in the bloody Battle of Bataan.

When the Japanese finally won the battle on April 9, 1942, it captured some 75,000 American and Filipino troops. These captured troops were already in poor physical shape from months of fighting under siege and were forced to march to a prison camp some 65 miles away. Rarely given any food or water despite the brutal conditions, any soldier who slowed was often killed on the spot. Filipino civilians who tried to offer any aid to the prisoners, most of whom were Filipino soldiers, were also frequently killed on the spot. When Americans learned of this brutality, they vowed to accept nothing but the total defeat of Japan.

December 1943: No Separate Peace

With wars in both Europe and the Pacific, concerns mounted about how to conduct the fighting.  After the Soviet victories in the Battle of Stalingrad in February 1943 and the Battle of Kursk in July 1943, putting the Germans on the defensive, the Allies had to decide on strategies for winning World War II in an acceptable manner. What would constitute a victory? Each Allied Power had competing goals and interests. Soviet Premier Joseph Stalin, US President Franklin D. Roosevelt, and British Prime Minister Winston Churchill met in Tehran, Iran between November 28 and December 1, 1943 to plot Allied strategy.

The conference reiterated that there could be no separate peace, where one ally signed an armistice with the Axis Powers and left the war. The fear was that the Soviet Union would sign an armistice with Nazi Germany once it had pushed the Germans out of Soviet territory, with Stalin frustrated at the high Red Army casualties from doing the vast bulk of the ground fighting.  In exchange for the US and Britain opening a second front against Germany by landing in Nazi-occupied France, the Soviets agreed to declare war on Japan within three months of Germany’s surrender. On August 8, 1945, the Soviet Union declared war on Japan and invaded Japanese-held territory in northern China. Historians debate whether the Soviet entry into the war or the American dropping of the two atomic bombs had a greater effect in leading to Japan’s acceptance of unconditional surrender on August 15.

Geographically, it was difficult for Nazi Germany and fascist Italy to meaningfully assist imperial Japan during the war. Fortunately, the Axis Powers were never able to link up forces on land, such as through India and the Middle East or in the Soviet Union. Thus, aid among the Axis Powers was largely limited to technical assistance and the occasional naval visit. Japan sought German technical assistance, especially after the United States trade embargo in 1941. After flights between Europe and Japanese-held territory in Asia were determined to be inefficient, submarine warfare was determined to be the area of most effective collaboration between Germany and Japan.

Occasionally, German and Japanese submarines met in the Indian Ocean, and in September 1943, a German submarine arrived in Japan to be studied as part of technical assistance. A Japanese submarine crew successfully trained in Germany but was lost at sea during the return trip home. Trading ships tried to operate between Germany and Japan periodically, but most were lost to the Allies. Late in the war, some German submarines were refitted as transport submarines to bring resources to Japan, but this had no effect on the war’s outcome.