The Washington-led ambush of China over the disputed South China Sea at the region's top security forum on Friday marks a landmark shift in Sino-US ties and exposes deepening strategic fault lines in Asia.
Even as US Secretary of State Hillary Clinton figuratively waded into the South China Sea in Hanoi, US and South Korean naval vessels prepared to stage large-scale exercises in the Sea of Japan, or East Sea, close to China's northeast - adding to the tensions of the new landscape.
What happened in Hanoi is particularly significant. When Clinton declared that resolving territorial claims in the South China Sea was now in the United States' 'national interest' and 'a diplomatic priority', she was not just reflecting growing US concern about the potential for Chinese maritime dominance. It showed Washington had firmly grasped an historic opportunity, too.
For months now, a rising chorus of East Asian concern at Chinese assertiveness has been voiced in Washington, just as the young administration of US President Barack Obama mapped out ways to re-engage with a neglected region. Alarmed by the refrain that the US was a declining power, US officials spoke privately of the need to reassert US strategic primacy in Asia.
China's increasingly strident assertions of its historic, and now legal, claim to virtually the entire sea - exemplified by its detention of hundreds of Vietnamese fishermen, the harassment of ships of the US and other navies and threats made to international oil giants aimed at ending their exploration deals with Hanoi - provide that opportunity.
The US move is not just about pleasing China's rival claimants to the sea's Spratly and Paracels archipelagoes - Vietnam, Malaysia, the Philippines and Brunei - but reassuring bigger players such as South Korea, Japan and Indonesia by sending a stark message to China.
For most of the past 15 years, Washington has kept firmly to the sidelines of tensions in the South China Sea, the strategic and mineral-rich waterway that links East Asia to the Middle East and Europe. Its envoys occasionally raised concerns about the need for a peaceful settlement of territorial disputes but took no side in the territorial disputes.
Clinton's remarks change all that. They put the US at the forefront of a Chinese sovereign issue - and one it recently stated was a 'core interest', diplomatic code that ranks it with Taiwan and Tibet for sensitivity.
Earlier this year, US Defence Secretary Dr Robert Gates told a security forum in Singapore that Washington objected to any effort to intimidate US oil firms engaged in lawful contracts in the region.
Clinton made her comments in the formal setting of the Asean Regional Forum, in bilateral meetings and in public statements. Her officials, meanwhile, briefed the travelling Washington press pack so they would not miss the point.
While she stuck to the old script about not taking sides, she made clear Washington wanted to foster regional discussions and solutions - a direct challenge to Beijing, which had tried, discreetly but forcefully, to scotch Asean discussion of the issue and whose envoys have insisted it should be handled bilaterally - in other words, by having individual claimants line up to cut their own deals with China.
'The United States supports a collaborative diplomatic process by all claimants for resolving the various territorial disputes without coercion,' Clinton said. 'We oppose the use or threat of force by any claimant.'
Her words were a significant diplomatic victory for Vietnam - a gift from Washington as the two countries mark 15 years since their formal restoration of diplomatic ties following the Vietnam war and a near-20-year economic embargo.
For months, Vietnam has been seeking to exploit its turn as chairman of the 10-nation Association of Southeast Asian Nations to keep the South China Sea issue on the boil. It is desperate to make progress on a legally binding code of conduct for all claimants to the sea's riches - a pledge made in a 2002 declaration signed between Asean and China on the South China Sea calling for self-restraint. The declaration was initially hailed as a significant step forwards, but increasingly appeared a dead letter in the face of China's actions.
Clinton repeatedly referred to the principles of that declaration, music to the ears of Vietnamese officials.
Just a year ago, China was widely seen as having split Asean, with each member putting its own relations with Beijing before Asean unity. There was little momentum on the issue in formal meetings. And in the background, Chinese pressure was constant, according to several Asean diplomats. Even Cambodia, whose regime was once closely allied to Hanoi, was quashing Vietnam's efforts on behalf of Beijing.
That caution was still visible hours before Clinton's arrival. On the eve of the Asean Regional Forum, the bloc's foreign ministers had their formal annual meeting with Foreign Minister Yang Jiechi . Only the Philippines raised the South China Sea issue. Such reticence reflects the traditional Asean way. Formal meetings and statements are generally as bland as possible.
Even Vietnam rarely rebukes China in public, preferring to maintain the facade of fraternity. Yesterday, flush with a rare victory, its ever-cautious state press stuck to official blandishments about the meetings.
'It was remarkable,' one diplomatic observer said. 'No one wanted to lead from the front ... they were all waiting for safety in numbers.'
Clinton's arrival on Thursday appeared to provide that sense of safety, as word spread of her new firm line.
By the time the forum started on Friday, 11 other members were ready with statements, including Brunei, Malaysia, the Philippines and Vietnam - those with most at stake - as well as Indonesia, the EU, Australia and Japan. What followed was a rarefied form of tag-team wrestling.
While Yang expressed exasperation afterwards, what happened should not have come as any surprise to Beijing. For more than a year, military, political and diplomatic manoeuvres have pointed to the growing concerns in the region.
Once-reticent Vietnamese military chiefs have been flown out to US aircraft carriers in the South China Sea and been invited aboard US submarines in Hawaii. And Vietnam has allowed US warships to be repaired in local ports.
Hanoi has also struck a deal with former cold war ally Moscow to buy six state-of-the-art Kilo class submarines.
Pentagon and US State Department officials, meanwhile, have been increasingly explicit in testimony to the US Congress about the need to assert US navigational rights in international waters in spite of China's concerns. While China claims much of the South China Sea as its exclusive economic zone, the US and other nations insist that it is still in international waters and thus that routine military operations, including surveillance, are allowed.
The tensions this creates bubbled to the surface when Gates spoke in Singapore to an audience that included senior officers of the People's Liberation Army.
'We are not treating it as a 'Chinese lake', we do allow innocent passage,' one PLA official fumed afterwards. 'But I'm sorry, US surveillance is not innocent passage ... the concern of China must not be underestimated.'
Whether or not Washington has underestimated those concerns remains to be seen. Undoubtedly Beijing will see the events in Hanoi as a considerable provocation. There is a growing sense in the region, too, that the sea is vital to China's ambitions for a 'blue water' navy able to operate far from its shores, since it provides its only deep-water gateway to the Pacific and Indian oceans.
What is clear is that the US is now prepared to tackle one of the region's most intractable problems - a policy shift that will not be easy to back away from. And for China, any Asean honeymoon is over.
For all the risks, Washington senses opportunity.