He did this work in association with American director Arthur Rankin, Jr. who wrote and designed the productions before sending them to Japan for animation.
During this time, he was the assistant of Seo Mitsuyo on Momotarō's Sea Eagles, an animated propaganda film depicting the bombing of Pearl Harbor.
"[2]Shortly after the release of Momotarō's Sea Eagles, Mochinaga was put in charge of Fuku-Chan's Submarine despite his own protests that he was too inexperienced.
Due to the fact that much of his staff was drafted to the military and lack of supplies and funds, Fuku-Chan's Submarine was barely finished and released in 1944.
Foreseeing the end of the war and fearing the inevitable American occupation of Japan that might involve purging propagandists as well as food shortages, Mochinaga fled with his pregnant wife to Japan-occupied Manchuria where they both had family.
Mochinaga soon found himself in another war zone as the Chinese National and Communist armies battled for dominance over Manchuria.
In 1946, the frontline of China's Civil War moved further south, allowing Japanese nationals to finally return home.
Mochinaga continued to work as a successful animator and filmmaker in China for the next decade, finally returning to his home country in 1954, four years before the massive famines in 1958.