After 1930, the extreme nationalism fostered by the Meiji and Showa Imperial governments 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. Hard-line militarists in the Japanese government and army pointed to Japan's samurai military traditions, and accused moderate politicians, bureaucrats, and armed service leaders of disregard for Japan's national interests if they opposed military spending or territorial expansion by force. The views of hard-line militarists became increasingly influential in shaping Japan's foreign policy during the 1930s, and they used intimidation and assassination of moderate politicians, bureaucrats and armed service leaders as means to achieve their aims.

In 1931, Japan invaded China's vast northern region of Manchuria and converted it into a Japanese puppet state called Manchukuo. Japanese armies then invaded China's Jehol province which adjoined the former Chinese Manchuria.

In 1933, Japan formally incorporated 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.

A more detailed treatment of Japanese aggression during the 1930s can be found in the section "Imperial Japan's Path to World War II".

Despite the seizure of so much Chinese territory, the militarist dominated Imperial government was still committed to further expansion of Japanese territory by military force. In pursuance of this aim, the Imperial government formulated the following major strategic 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, when opportunities arose, further territorial expansion to the south should be undertaken to seize for Japan the food and raw materials in the South-East Asian colonies of Britain, France and the Netherlands.

The Imperial Japanese Navy was more cautious about aggression towards Japan's neighbours in the western Pacific, especially if the aggression might bring about conflict with the United States Navy. A rising naval officer named Isoroku Yamamoto believed that Japan could not win a drawn-out war against the United States, and he supported the 5:5:3 naval tonnage ratio for Great Britain, the United States, and Japan respectively as being the best way to avoid armed conflict between Japan and the United States.

During tours of duty in the United States, Yamamoto observed American demonstrations of the effective use of carrier-launched naval aircraft against warships, and became convinced that aircraft carriers were more powerful naval weapons than battleships. He became an influential advocate for the use of aircraft carriers as primary naval strike weapons. Yamamoto would later initiate and plan the devastating carrier-based air attack on the United States Pacific Fleet at Pearl Harbor.

The Japanese Army never accepted the 5:5:3 naval ratio imposed by the Washington Naval Conference, believing that it symbolised Japan's humiliation by Great Britain and the United States. By the middle of the 1930s, the Army's hostility to the naval limitation treaty had been adopted by most Japanese. Having acquired new allies in Nazi Germany and Fascist Italy by joining the Anti-Comintern Pact in 1936, Japan withdrew from the naval limitation treaty in 1937. Japan then began to expand its navy, with particular emphasis on building aircraft carriers and huge battleships, such as the Yamato and Musashi, which were twice the tonnage of the largest British and American battleships.

In July 1937, tensions between Chinese troops and Japanese troops engaged in military exercises on occupied Chinese territory produced an exchange of firing near Peking (now Beijing). The Japanese used this incident as an excuse to wage a brutal undeclared war against China that lasted until 1945. Japanese armies invaded China's northern provinces and quickly captured the former Chinese capital Peking (now Beijing). Although poorly trained and equipped, the Chinese army and communist irregulars put up strong resistance to Japan's armies which enjoyed overwhelming superiority in numbers and weapons. Although the main purpose of the war was to seize the whole of China, the Japanese Navy and Army took advantage of its lengthy duration to test new weapons and tactics against the Chinese, and to provide their airmen and troops with combat experience. Before the war ended in 1945, between five and ten million Chinese would die at the hands of the Japanese.