The recent tripartite agreement between China, Vietnam and the Philippines for joint exploration of the Spratlys in the South China Sea should be a concern to all of Asia. The agreement is a breach of political solidarity with the 10-member Association of Southeast Asian Nations, whose influence has been key to containing Chinese hegemonic expansion in the region.
Beijing can easily deploy its "grab and talk" tactics with individual claimants, but not if the claimants bind themselves together and deal with China as a group through the ASEAN framework. Once ASEAN unity is broken, China can build up its presence in the region, as it did when it put up a fortress in Mischief Reef in the late 1990s.
Tokyo should be concerned about this matter because Chinese control of the Spratlys would be a threat to the economic security of Japan and South Korea. Seventy percent of the oil exports to both countries pass through the sea lanes between the Spratlys and Palawan. Surely, allowing one power to control these sea lanes would disrupt the balance of power in the Far East.