Japan was unable to send missions to Tang China through the northern route and had to find a new route, because Japan lost its base foot in the Korean Peninsula in the Battle of Baekgang in 663 and Silla drove the Tang army out of the peninsula to unify its country in 676.
663年の白村江の戦いで倭国は朝鮮半島での足場が無くなり、676年に新羅が半島から唐軍を追い出して統一を成し得たため、倭国は北路での遣唐使派遣が出来なくなり、新たな航路の開拓が必要になった。