HONG KONG/HANOI (Reuters) - The visit of a US aircraft carrier to Vietnam for the first time since the end of the Vietnam War is a powerful symbol of the growing strategic ties between the former foes.
But the arrival on Monday of the USS Carl Vinson also illustrates Hanoi's complex and evolving relationship with Beijing over the disputed South China Sea.
For months now, Vietnamese envoys have been working to ease the concerns of their giant Chinese neighbour over the visit and the prospect of broader security co-operation between Hanoi and Washington, according to diplomats and others familiar with discussions.
Vietnamese diplomats and military officers have repeatedly stressed the country's independent foreign policy and its desire for broad foreign relations - hoping to maintain stable ties with China while standing up to it over the South China Sea, the sources said.
The Vinson will mark the biggest US military presence in the country since 1975 when it berths in Danang for a five-day stay.
The port city on Vietnam's central coast is close to its Blue Whale gas field now being developed by US oil major Exxon Mobil, as well as the increasingly fortified Paracel islands, which China occupies and Vietnam also claims.
China's rapid construction and build-up of the seven features it holds in the disputed Spratly group further south has alarmed Vietnam and other regional governments.
As it seeks to enforce its claims to much of the South China Sea, China's navy and coastguard now routinely patrol vast swathes of the area through which some US$3 trillion in trade passes annually.
While some Chinese commentators have used the Vinson's presence to demand an even greater Chinese military build-up in the waterway, official reaction from Beijing has been relatively muted since the stop was confirmed in January.
US carriers frequently ply the South China Sea as part of a rising pattern of naval deployments, and are now routinely shadowed by Chinese naval vessels, regional naval officers say.
"Even as it moves forward with the US, Vietnam has been engaging China over South China Sea tensions," said Le Hong Hiep, a Vietnam expert at Singapore's ISEAS Yusof Ishak Institute.
"They might be irritated but gradualism is the key word here...Vietnam will only embrace the U.S. at a pace that doesn't cause an over-reaction from China."
Zhang Baohui, a mainland security expert at Hong Kong Lingnan University, said he believed Beijing would not be panicked by the carrier visit, in part due to quiet but successful Vietnamese diplomacy.
"Beijing has a much better understanding of Vietnam's balancing formula now," he said.
The Vinson arrives amid repeated signals from the administration of US President Donald Trump that it is keen to further develop security ties with Vietnam as part of a range of political and military relationships to check the rise of China.
Given historic Vietnamese suspicions of Beijing, Hanoi's Communist Party leadership has forged a range of security relationships to balance its own ties with China.
Moscow, its major patron through the Cold War, remains the key supplier to the modernising Vietnamese military, and India and Israel are increasingly important suppliers.
Trump himself touted possible missile and other advanced weapons sales during his November visit to Vietnam but not major arms deal has yet been completed.
His administration's National Security Strategy, published in December, listed Vietnam as a "co-operative maritime partner".
Derek Grossman, a senior defence analyst at the US RAND think-tank, said it was uncertain just how fast and how far Vietnam was prepared to go even as it sought to deter China in the South China Sea.
"It seems the US side is still not sure quite what Vietnam wants, but the focus will likely be on ways to enhance Vietnam's maritime domain awareness and maritime security capabilities,"he said.
A recent study of Vietnam's military modernisation by Grossman, published by the Seattle-based National Bureau of Asian Research, found that Hanoi would struggle to sustain a large-scale conflict if its deterrence strategy failed. Despite the acquisition of its first submarines, new Russian jet fighters and missiles, Vietnam faced extensive deployment and integration hurdles.