Watanabe attended military preparatory schools in Osaka and was a graduate of the 21st class of the Imperial Japanese Army Academy in 1909, specializing in artillery.

Advancing north through the Shan States the 56th Division defeated elements of the Chinese 65th Corps to take the city of Lashio on the Burma Road.

The fall of Lashio to the 56th Division on 29 April 1942 cut off much of the local Chinese Army from China and compelled the Allies to evacuate Burma.

However, with the war situation rapidly deteriorating against Japan, he was sent to the field again in March 1944 as the first commander in chief of the newly formed IJA 32nd Army, tasked with the defense of the Ryukyu Islands.

In the final stages of the war, he was recalled to duty, and assigned as commander of the Osaka region for the expected American invasion of the Japanese home islands.