While Japan was giving the appearance of being a good neighbour in the western Pacific during the 1920s by involvement in treaties designed to preserve peace, extremist elements in Japan's government, military and civilian population had privately never renounced the use of force to expand Japan's territory. Towards the end of the 1920s a combination of economic, social, and political factors played into the hands of the militarists.

Having been barely touched by World War I, Japanese industry and trade had expanded dramatically during that war to fill the gap left by Europe's devastated industries. However, most of the raw materials needed to supply Japanese manufacturing industry had to be imported because Japan possessed inadequate natural resources. This problem was compounded by substantial population increase. Between 1918 and 1930, Japan's population had expanded dramatically and outstripped the capacity of the nation's resources to support it. To sustain its population blow-out, substantial food imports were essential, but foreign tariffs imposed on its exports of manufactured goods limited the capacity of Japan to pay for its food imports. Japan had tried to deal with its population problem by encouraging emigration of Japanese to countries such as the United States, but had met resistance from Americans who feared the loss of unskilled jobs to cheap immigrant labour.

Invading Japanese troops enter China's ancient capital Peking in July, 1937. With a view to seizing the whole of China, Japan waged a brutal and unrelenting war against the Chinese until 1945. Millions of Chinese were killed.

With China torn by revolution in the 1920s, Japan's militarists viewed China, and in particular, its resource-rich northern region of Manchuria, as an obvious area for Japan to expand its territory by military force and thereby solve its raw material and population problems. However, the Japanese imperial government was not responsive to proposals for military aggression against China at this time.

During the 1920s, Japanese militarists became increasingly distrustful of civilian party government. In 1922, the Washington Naval Conference had allocated to Japan a smaller naval tonnage than that allowed to Great Britain and the United States. This caused resentment in Japan, particularly in the ranks of army and civilian militarists who viewed it as a humiliation for Japan. In 1925, the Kato imperial government cut the army and navy budgets and reduced the Japanese Army by four divisions.

Between 1925 and 1928, Chinese nationalists under the banner of the Kuomintang (Nationalist Party) had begun to unite all of China's far-flung regions under a Kuomintang Nationalist government. Japan's militarists feared that a unified China under a Nationalist government would block Japanese territorial expansion into Manchuria where Japan had massive commercial interests and had achieved significant political influence. In 1927, Japanese militarists demanded action by the imperial government to block the Chinese Nationalist movement reaching Manchuria. The militarist Prime Minister Tanaka responded by sending Japanese troops to China's Shantung province in 1928 to block a union of Manchuria with the Chinese Nationalist cause.

Military extremists take control of Japan's foreign policy

When Japan acquired Port Arthur on the Kwantung Peninsular of southern Manchuria as one of the fruits of its victory in the Russo-Japanese War of 1904-1905, a Kwantung Army was established to occupy the peninsula, patrol the Japanese-owned South Manchuria Railway zone, and generally protect other Japanese commercial interests in Manchuria. The Kwantung Army included extremist officers who were well aware that senior officers of their army harboured plans to extend Japan's boundaries on the Asian mainland by military force. They were prepared to take direct action in Manchuria to force the Imperial government's hand

When Manchuria embraced Kuomintang nationalism in 1928, friction quickly developed between Chinese nationalists and Japan's pervasive and heavy-handed bureaucrats in Manchuria. The Manchurians wanted to reduce Japan's political influence in their region of China to a purely commercial presence. They also began to develop Chinese-owned railways to compete with the Japanese-owned and controlled South Manchuria Railway. Japan's militarists viewed these developments as threatening Japan's "special position" in Manchuria and their plans to seize for Japan this huge northern region of China. Extremist officers in Japan's Kwantung Army took steps intended to dampen Nationalist enthusiasm by assassinating the Chinese warlord ruler of Manchuria in 1928, but his successor was an even stronger supporter of Chinese nationalism. The murder of the Manchurian ruler was not authorised by the imperial government, but when the Tanaka government tried to punish the culprits and re-establish discipline in the army, it was blocked by the Japanese Army General Staff.

The failure to punish the murder of a foreign political leader by officers of the Kwantung Army in Manchuria clearly demonstrated that the imperial government had lost control of extremists in the Japanese Army, and the Tanaka government felt obliged to resign in July 1929. From this time onwards, the course of Japanese foreign policy became increasingly hostage to army extremists, and imperial governments and the Japanese Army General Staff were either unwilling or unable to curb them.

Intimidation and assassination destabilise Japanese politics

After 1929, the extreme nationalism encouraged by the Meiji imperial government combined with traditional Japanese militarism to make life increasingly difficult, and often dangerous, for moderates in the imperial government, the Diet (parliament), and the armed services. Army and civilian extremists pointed to Japan's samurai military traditions, and accused moderate bureaucrats, politicians, and armed service leaders of disregard for Japan's national interests if they opposed increased military spending or territorial expansion by force. Extreme nationalists branded democratic government as "un-Japanese", and called for territorial expansion and a return to traditional Japanese ways. The views of extremists would become increasingly influential in Japan during the 1930s, and they would use intimidation and assassination of politicians, businessmen and armed service leaders as means to achieve their aims.

The militarist-dominated Tanaka imperial government was followed by the more moderate Hamaguchi government which tried to curb the power of military extremists. The extremists responded by plotting to overthrow civilian government, and Prime Minister Osachi Hamaguchi was assassinated in 1930. In March 1931, a coup involving senior military leaders was planned but abandoned. Within months, events in Manchuria would draw the focus of Japan's turbulent politics from Tokyo to that vast northern region of China.

Militarists launch Japan on the path of aggression by seizing Manchuria from China, 1931

Japan's economy was seriously affected by the Great Depression which began in 1929, and with revenues from Japan's commercial interests in China's Manchurian region thought to be under threat from Chinese nationalism, military and civilian extremists found Japan's imperial government now willing to listen to their demands for a move against Manchuria.

In 1931, militarists dominated the imperial government, and all that they required was a plausible excuse for military action in Manchuria. Rather than wait for such an excuse to occur, the Kwantung Army extremists appear to have decided to create one. On the night of 18 September 1931, a bomb was exploded on the track of the Japanese-owned South Manchuria Railway. The explosion caused very little damage, and no loss of life. The Kwantung Army immediately blamed "Chinese terrorists", and without waiting for approval from the imperial government in Tokyo or producing any proof of its allegation, its troops seized the Manchurian city of Mukden. Proclaiming a need to protect Japanese life and property, and again without approval from Tokyo, the Kwantung Army then undertook the full conquest of Chinese Manchuria. The Kwantung Army simply ignored efforts by the imperial government in Tokyo to bring its military aggression in Manchuria to a halt. When Prime Minister Inukai tried to secure Emperor Hirohito's intervention to bring the Kwantung Army back under government control, he was assassinated by naval officers in May 1932.

When the Kwantung Army had completed its conquest of Manchuria, it converted this vast former region of China into a Japanese puppet state called Manchukuo in September 1932. The last Manchu emperor of China, Henry Pu Yi, agreed to be enthroned as emperor of Japan's puppet state, and he ruled Manchukuo under the control of the Kwantung Army.

Japan occupies and annexes China's Jehol Province in 1933

China complained to the League of Nations which called on member states to withhold recognition of Manchukuo. Japan used this mild reprimand as an excuse to withdraw from the League, and freed from the restraints of the League's Charter, Japanese armies then invaded areas of northern China adjoining the former Chinese Manchuria. Japanese troops occupied China's northern Jehol province and stopped short of the former Chinese capital Peking when a truce was arranged. Under the terms of the truce, Chinese troops were barred from the areas of northern China occupied by Japanese armies. In 1933, Japan formally incorporated China's Jehol province into its puppet state Manchukuo. With two hostile armies facing each other on Chinese territory, the Japanese militarists had set the stage for further conflict with China when a suitable pretext occurred.

Army extremists attempt to overthrow Japan's Imperial Government in 1936

By the mid-1930s, army extremists had become impatient with Japan's existing political and economic structures which they felt were impeding Japan's progress towards military dominance of Asia. They resolved to destroy the power of the politicians and industrialists who were the emperor's chief advisers. On 26 February 1936, fanatical army officers assassinated two of Emperor Hirohito's key advisers, and army mutineers surrounded the Japanese Foreign Office and held much of Tokyo city for three days. Prime Minister Keisuke Okada escaped the assassins' bullets when they killed his brother-in-law by mistake. The plot to overthrow civilian government failed when the Army High Command refused to support the mutineers. The leaders of the mutiny were persuaded to commit suicide to avoid a trial which would have embarrassed the army. The most extreme military leaders were then replaced by ones who were prepared to support civilian government.

Japan's Foreign Ministry in Tokyo surrounded by army mutineers, 1936.

Despite the failure of the army mutiny, the imperial government was still dominated by militarists and committed to extension of Japan's borders by military force. In pursuance of this aim, the imperial government formulated the following major foreign policy objectives for Japan: Russian pressure on Japan's empire from the north needed to be resisted; the military conquest of the whole of China should be undertaken; and further territorial expansion to the south should be undertaken to seize for Japan the wealth and raw materials available in the South-East Asian colonies of Britain, France and Holland.