US aircraft carrier Carl Vinson is making a historic call at Vietnam, the first time a ship of this size has visited since the Vietnam War ended.
The nuclear-powered carrier set anchor off the port city of Danang, where US combat troops first landed in the war, making this a highly symbolic location.
The visit is meant to demonstrate the countries' growing military ties.
But analysts say it inevitably sends a message to China as it continues to develop the disputed South China Sea.
It comes as China announced a military budget of 1.11 trillion yuan, an 8% increase on last year, as its annual meeting of parliament got under way in Beijing.
The BBC's Jonathan Head, who is in Danang, says that while co-operation between the US and Vietnam's military has grown it is still limited, and Vietnam has to manage the messaging of this visit carefully.
China is now the de facto superpower in this region, and by far the most important trading partner for Vietnam.
So Vietnam's communist leadership is taking care to avoid any steps that might upset relations with its giant neighbour, our correspondent says.
China claims almost all of the South China Sea, including reefs and islands also claimed by other nations. Vietnam specifically exercises claims over the Paracel and Spratly islands.
The US has always said it does not take sides in the South China Sea dispute, but the US Navy has consistently run so-called "freedom of navigation" operations through disputed waters, a clear challenge to China's territorial ambitions.
The USS Carl Vinson has made many journeys to the wider region in its decades of service and was last in the sea just days ago.
Danang was a major US military base during the Vietnam War. The docking of the vast vessel, which can support up to 90 aircraft, will represent the largest US military presence in Vietnam since the end of many years of bitter conflict and after the country was unified in 1975.
The Vietnam War - which Vietnam calls the American War - was protracted and bloody. The Vietnamese government estimates that millions of Vietnamese, both civilian and Communist fighters, were killed. Just over 58,000 American soldiers were killed or went missing in action.
Only a symbolic message: Jonathan Head, BBC News, Danang
The presence of a US aircraft carrier in Danang, one which will be conducting so-called "freedom of navigation" passages through the disputed areas of the South China Sea, also sends a potentially provocative message to China - but only a symbolic one.
To date US expressions of concern over China's actions in the sea have had no discernable impact on the construction of military facilities there.
The US carrier strike group looks impressive, but it is not clear what it can actually do to restrain China. Even as Vietnam's top brass enjoy watching the full might of the superpower they once fought now paying court as a would-be ally, they must remain mindful of the continuing historic shift in the balance of power in their region away from the United States, in favour of China.