It was considered that when the Japanese government found out about the upcoming war in Silla between Tang China and Bokkai, it decided to participate in the war, but before that it managed to evacuate Japanese students and monks, who were left behind in Tang China after the previous envoys came back to Japan.
新羅で唐と渤海の開戦が近いことを知った日本は参戦の決意を固め、その前に前回の遣唐使が残した留学生や留学僧を帰国させようとしたと考えられる。