Vietnam has become the latest focus in the U.S.-China contest for the hearts and minds of Southeast Asian nations.

Now U.S. ally Japan has joined the fray by offering defense cooperation with Vietnam. But this effort is unlikely to go far or deep in drawing Vietnam to the allies’ side. Indeed, Vietnam is China’s to lose.

Both the U.S. and China see Vietnam as a key claimant in the South China Sea. If either can stick and carrot it to their side, they think the rest will follow. This makes Vietnam a critical player.