During the Boshin War, he fought on the side of the pro-Tokugawa Ōuetsu Reppan Dōmei against the forces of Emperor Meiji, including the Battle of Hakodate.

With the outbreak of the Russo-Japanese War, Umezawa was given a battlefield promotion to major general in July 1904; however, despite his long history of military service, Umezawa was regarded as a mediocre soldier by his superiors, and their lack of expectation was reflected by the fact that he was given command of the Guards Reserve 5th Infantry Brigade.

This was a rear-duty garrison force composed primarily of over-age reservists armed with obsolete equipment (some of which was captured Russian weapons) and with low morale.

Umezawa defied all expectations by improving morale and turning his second-rate force into a first-rate combat unit.

Despite an order to withdraw, Umezawa's reserve brigade held for three hours against three times their number of Russian front-line troops.