He graduated in the 30th class of the Imperial Japanese Army Academy in 1918, and was assigned to the 16th Infantry Regiment, based at Shibata city in the Hokuetsu region of Japan.

He was then assigned to Army Intelligence, where he oversaw the wiretapping of foreign embassies, interception of mail and correspondence and the production of counterfeit money for use in future operations.

In 1937, he was transferred to the newly created 8th Section of the Imperial Japanese Army General Staff, and was tasked with planning the independence of Wang Jingwei's Nanjing Nationalist Chinese government.

With the defeat of the Japanese Army at Nomonhan Iwakuro became more outspoken in favor of the concept of a Greater East Asia Co-Prosperity Sphere and the need for Japan to prepare for total war.

But, according to John Toland, The Rising Sun, volume one, page 85, "The former (previous sentence referenced Matsuoka) suggested that Ikawa sound out the army in the person of an influential colonel in the War Ministry named Hideo Iwakuro.

[2] After the start of the Pacific War, the 5th Imperial Guards Regiment came under the command of the Southern Expeditionary Army Group, and was assigned to Malaya and Singapore.