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.