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Early service as officer

Upon graduating from the Japanese Military Academy (ranked 10th of 363 cadets) in March 1902, he was commissioned as a second lieutenant in the infantry of the IJA. In 1918–19, he briefly served in Siberia as part of the Japanese expeditionary force sent to intervene in the Russian Civil War. He served as Japanese military attache to Germany between 1919 and 1922. As the Imperial Japanese Army had been trained by a German military mission in the 19th century, the Japanese Army was always very strongly influenced by intellectual developments in the German Army, and Tojo was no exception. In the 1920s, the German military favored preparing for the next war by creating a totalitarian Wehrstaat (Defense State), an idea that was taken up by the Japanese military as the "national defense state." In 1922, on his way home to Japan, he took a train ride across the United States, his first and only visit to North America, which left him with the impression that the Americans were a materialistic soft people devoted only to making money and to hedonistic pursuits like sex, partying, and (despite Prohibition) drinking.

Tojo boasted that his only hobby was his work, and he customarily brought home his paperwork to work late into the night and refused to have any part in raising his children, which he viewed both as a distraction from his work and a woman's work. He had his wife do all the work of taking care of his children. A stern, humorless man, he was known for his brusque manner, his obsession with etiquette, and for his coldness. Like almost all Japanese officers at the time, he routinely slapped the faces of the men under his command when giving orders. He said that face-slapping was a "means of training" men who came from families that were not part of the samurai caste and for whom bushido was not second nature.

In 1924, Tojo was greatly offended by the Immigration Control Act, which was passed by the US Congress. It banned all Asian immigration into the United States, with many representatives and senators openly saying the act was necessary because the Asians worked harder than whites. He wrote with bitterness at the time that American whites would never accept Asians as equals: "It shows how the strong will always put their own interests first. Japan, too, has to be strong to survive in the world."

By 1928, he was bureau chief of the Japanese Army and was shortly thereafter promoted to colonel. He began to take an interest in militarist politics during his command of the 8th Infantry Regiment. Reflecting the imagery often used in Japan to describe people in power, he told his officers that they were to be both a "father" and a "mother" to the men under their command. Tojo often visited the homes of the men under his command, assisted his men with personal problems, and made loans to officers short of money. Like many other Japanese officers, he disliked Western cultural influence in Japan, which was often disparaged as resulting in the ero guro nansensu ("eroticism, grotesquerie and nonsense") movement as he complained about such forms of "Western decadence" like young couples holding hands and kissing in public, which were undermining traditional values necessary to uphold the kokutai.