The phase following the First World War saw the complete affirmation of the Japanese Empire as a great power: after having absorbed part of the German colonies of the Pacific Ocean and having assumed control of several lucrative trade routes in the basin, with the Washington Naval Treaty of February 6, 1922 Japan obtained the right to have the third largest battle fleet in the world, a condition that guaranteed it military superiority since its strongest contenders (the United States and the United Kingdom) had to divide their fleets between Pacific and Atlantic. The outbreak of the great depression in 1929 prompted the country to change its economic focus, previously concentrated in trade with the United States, and to look more closely at the Asian markets; excluded from the colonial partitions of the 19th century, Japan felt deprived of access to the rich resources of Asia by the European powers and decided to compensate for this state of affairs with a series of aggressive maneuvers of territorial expansionism

Japan’s slide towards a policy of imperialism was favored by a strong militarization of Japanese society, which had already begun in the mid-twenties: the pervasiveness of the military, capable of influencing national political life through the actions of the powerful secret police forces (the Tokubetsu Kōtō Keisatsu) and military (the Kempeitai), became exemplary in the field of education of the new generations, through the assignment as teachers in public schools of numerous army officers left without assignments. The influence of the military in society led to the recovery of the medieval philosophical concept of Gekokujō, according to which a lesser officer can disobey superior orders if he deems it morally right; in addition to degenerating into a series of bloody but unsuccessful coup attempts by ultra-reactionary officers (such as the incident of February 26, 1936), this principle was the justification adopted by the Japanese generals to carry out campaigns of territorial expansionism in a all autonomous from the wishes of the national government proper

Japanese troops occupy Beijing in August 1937

The primary outlet for this expansionism was China, weakened by a ten-year civil war which pitted Mao Zedong’s communist forces against those of Chiang Kai-shek’s nationalist Kuomintang. Acting independently of the government, the Japanese generals orchestrated a mock railway sabotage at Mukden on September 18, 1931, used as a pretext to launch the invasion of the Manchuria region in northern China where the puppet state of Manchukuo was established. The occupation of Manchuria led to a state of deep diplomatic and military tension between Japan and the Soviet Union, which degenerated into a series of border skirmishes that continued until September 1939; this led to a diplomatic rapprochement between Japan and Nazi Germany in an anti-Soviet key, formalized with the signing of the Anti-Comintern Pact on November 25, 1936. The conflict between the Japanese and the Chinese finally exploded into total war starting in July 1937: the Japanese forces gave the the invasion of central and southern China, occupying Beijing and Nanjing within a few months but then found themselves embroiled in a long guerrilla conflict, in particular after the stipulation of a formal anti-Japanese alliance between Mao’s communists and the Chiang Nationalists; victory in the long war against the Chinese was therefore the cornerstone of Japanese foreign policy at the time of the outbreak of hostilities in Europe