During the later period of the First World War, the Empire of Germany declared unrestricted U-boat operations and attacked transport ships indiscriminately.

In December 1917, after the safety of Australia was secured, Toyoda returned to Japan and re-entered the Naval Staff College where he studied for two years as a first-class student.

He lived in London for four years, during which time he was promoted to captain, and handled intelligence collected by paid-spy William Forbes-Sempill, Master of Sempill.

Due to his extensive period abroad, Toyoda had become an expert on foreign affairs but knew little of the domestic situation within Japan and therefore often had conflicting views with the other members of his delegation at the disarmament talks.

It is supposed that the dismissal was caused by something Toyoda said to Prince Fushimi Hiroyasu, who had been newly appointed as head of the Imperial Japanese Navy General Staff.

As Toyoda’s grasp of aircraft technology slowly increased, he became dissatisfied with factory operations, as it was difficult to obtain the necessary tools and parts.

After careful consideration, Toyoda decided to leave the Imperial Japanese Navy; however, his letter of resignation was not accepted and he was promoted to full admiral and transferred to reserve duty instead.

After only three months, a cabinet reshuffle became necessary due to the forced resignation of the increasingly strident pro-Axis Foreign Minister Yōsuke Matsuoka.

Toyoda, who had earlier opposed the Tripartite Pact and was vocal in this opposition to prospects for war with the Soviet Union, replaced Matsuoka as Minister of Foreign Affairs on July 18, 1941.

One of his first priorities was to attempt to smooth over the rapidly deteriorating diplomatic relations with the United States, and he dispatched Admiral Kichisaburō Nomura as ambassador to Washington DC.

Toyoda improved work methods and initiated welfare programs for children who had graduated from junior high school and for Korean workers as well as for those who had lost their jobs.

After the surrender of Japan, most members of the wartime Japanese cabinets were accused of war crimes by the Allied occupation authorities and arrested.