Yui developed a reputation as a competent officer, and as a colonel at the start of the Russo-Japanese War, served as Vice Chief of Staff under General Yasukata Oku of the Japanese Second Army.

Towards the end of the war, he distinguished himself at the Battle of Mukden as Chief of Staff of the IJA 8th Division.

In April 1919. he dispatched Major General Yasutaro Takayanagi on a secret mission to survey Outer Mongolia as part of Japan's plans to encourage Mongolian separatism and to eventually detach that region from Chinese hegemony.

[1] In November 1919, Yui become a full general and was in command of the Japanese garrison at Tsingtao from 1919 until 1922.

Resigning from active duty in 1923, he was promoted to the honorific title of Junior Second Court Rank.