Asean secretary general Le Luong Minh's public comment that the Association of Southeast Asian Nations must "get [China] out of the territorial waters of Vietnam" has upset the apple cart of Asean neutrality - one of its most sacred assets.
Indeed, it is now clear that the China-US rivalry for regional dominance is rending Asean asunder and making untenable its "centrality" in regional security.
These remarks by Vietnam's former deputy foreign minister are of course unacceptable to China - and hopefully to the intellectual leaders and architects of Asean.
The association has so far studiously avoided blaming China publicly, specifically and directly for the rising tensions, despite lobbying by both Vietnam and the Philippines to do so. But there is little doubt Asean is divided regarding how to deal with China in the South China Sea, and on the more fundamental issue of China versus the United States.
Asean's neutrality is enshrined in its 1971 declaration of the region as a "zone of peace, freedom and neutrality free from any form or manner of interference by outside powers". But this polite fiction has been revealed as an ideal that is no longer real, if it ever was.
In a directive delivered by its foreign ministry spokesman Hong Lei, China demanded that "Asean abides by its neutral position, doesn't get involved in certain disputes, takes practical actions to protect the development of China's and Asean's relationship".
Beijing blames the US for encouraging Vietnam and the Philippines to provoke it regarding the South China Sea issues. Worse, Japan has joined in the China-bashing and China has essentially told it to mind its own business.
Australian analyst Hugh White explained China's recent actions by postulating that it may be trying to weaken American political influence in the region by pressing US allies and demonstrating that the US will do little or nothing to help them. If so, this could be a grave mistake. But it does put Southeast Asia and Asean uncomfortably between China and the US.
Southeast Asia attempted to avoid the repetition of its domination by outside powers by forming Asean in 1967. It began as a pro-West capitalist bulwark against the spread of communism and emphasised enhanced economic cooperation. Now it is trying to transform itself into a security organisation. But the China-US political and strategic struggle is making that difficult.
Unfortunately for Southeast Asia, this struggle is compelling countries in the region to choose between the two.
China basically sees itself as a victim, previously of colonial powers and now of their successors - small Southeast Asian countries who are stealing its maritime resources and are being backed by the US. This sense of victimisation feeds its nationalistic discourse and underpins its moral outrage and righteousness in foreign policy. Worryingly, it also reinforces public demand for tough and uncompromising positions.
The cracks in Asean are widening. Supposedly neutral players like Indonesia and Malaysia are now leaning towards the US. Singapore, as a "strategic partner", and the Philippines, a US ally, are already in the US corner.
Thailand is a holdover US military ally from another era. But if its behaviour during the second world war is any guide, it will bend towards the most powerful. Vietnam has been very public in its attempts to draw the US in as a balancer to China. The US has even made political inroads in Myanmar, heretofore a staunch China supporter.
But this US expansion has not erased the ancient influence of Chinese culture - and the respect and fear many feel towards China. Many Southeast Asian countries are fundamentally realistic and take the long view: China will always be there. Their valid concern is that the upstart US and its power, both soft and hard, may eventually recede like the outgoing tide, only to be replaced by a Chinese storm surge.
Asean was already divided on the South China Sea issues. Non-claimants Cambodia, Laos, Myanmar and Thailand seem to advocate deference to China. Brunei, Malaysia, the Philippines and Vietnam have claims that rival China - and each other.
Indonesia has attempted to play a mediating role between China, the Philippines and Vietnam. But now it has deviated from that line and directly criticised China's claims and actions.
In these circumstances, achieving a robust and binding code of conduct in the South China Sea is unlikely.
The more fundamental question for Asean is whether it can still maintain security in the region. Or will Southeast Asia be once again riven, manipulated, and tortured by a struggle between outside powers - this time between China and the US?
It is beginning to look like Asean unity in security is an unrealistic dream in the vortex of great power rivalry. I hope I am wrong.
Mark J. Valencia is an adjunct senior scholar at the National Institute for South China Sea Studies, Hainan