That was international politics. In Asia, only China could still organize effective resistance, in Europe, only England had the condition. So mainly the two nation got supports from outside, Russia could resisted by itself. China still had 3/5 territory including vast central, west and southwest area that could organize and supply huge amount of materials and sources of troops. From Sichuan province we provided dozens of divisions into the Shanghai battle, from Guangxi, Hunan the troops entered into Shangdong battle, from Xinjiang, we imported materials via Himalaya pass. And most universities, military schools had moved to southwest, they also could provide constantly professionals and military leaders....Help the nation they still had resisting will and ability. Japan occupied whole Vietnam in a week, how could outside power help Vietnam?
In the map, it is in fact the coastline islands. Nowadays you guys explain the map as to your demand, it doesn't work. Great Nam Empire's contribution to Vietnam is that they solidly annexed south Khmer Mekong area. Celebrate what should be celebrated.