By ending the ban, the U.S. is signaling its desire to leave behind decades of tense post-war relations with Vietnam and start a new phase of closer economic and military ties -- one facet of the "rebalance" toward Asia that's central to the president's foreign policy legacy.
At the same time, the U.S. is showing the region -- and particularly China -- that it is committed to maintaining international rules in Asia and to backing up smaller countries in area where tensions have been rising as an increasingly assertive Beijing tries to establish maritime claims in the South China Sea.
"The decision to lift the ban was not based on China or any other considerations," Obama said in Hanoi Monday. "It was based on our desire to complete what has been a lengthy process of moving towards normalization with Vietnam."
Obama added that part of U.S. cooperation with Vietnam is aimed at improving their maritime security posture "for a whole host of reasons," including strong defense ties. "But I want to emphasize that my decision to lift the ban really was more reflective of the changing nature of the relationship," he said.
The China factor
While Obama downplayed China, small countries in the region won't, said Bruce Klingner, a senior research fellow at the Heritage Foundation.
"Vietnam and other small nations in Southeast Asia are increasingly concerned by Beijing's actions and its push to extend its sovereignty" in the South China Sea, Klingner said.
In the last few years, China has become increasingly aggressive about its claims to maritime rights in the South China Sea, an area rich in fishing resources and, it's believed, oil and mineral reserves that lie beneath the seabed.
A slew of countries claim sovereignty over islands and waters in the area, including Cambodia, the Philippines, Malaysia, Taiwan, Brunei, China, Singapore and Vietnam. But China has gone a step further by building out tiny submerged reefs, turning them into man-made islands equipped with military-use facilities such as 3,000-foot runways -- a step that has heightened regional tensions.
Secretary of State John Kerry, traveling with Obama in Vietnam, underscored Tuesday the president's point that the U.S. is not targeting China, but supporting long-standing international norms.
"A China that assumes responsibilities as a global superpower leader and plays out its responsibilities in ways that are helpful -- that means encouraging peace and stability -- as long as China is playing by those rules and adopting it, none of this is focused on China," Kerry told reporters.
The U.S. emphasis on an international rules-based order that governs issues such as freedom of navigation and codes of conduct has been "the hallmark of American policy throughout the Cold War and beyond," Kerry said, "so it's not specifically focused on China."
Obama returned to that theme in Hanoi, saying that America and Vietnam "are united in our support for a regional order, including in the South China Sea -- where international norms and rules are upheld, where there is freedom of navigation and overflight, where lawful commerce is not impeded, and where disputes are resolved peacefully, through legal means, in accordance with international law."
The President added that the U.S. did not think disputes should be settled "based on who's the bigger party and who can throw their weight around a little bit more."
It would be too much to say that lifting the weapons ban was motivated solely by China's actions, said Sandy Pho, a program associate at the Kissinger Institute on China at the Wilson Center in Washington, adding that the key driving force behind the decision was Obama wanting to signal to the world that Vietnam is a key strategic partner. "But China is definitely a factor."
"Whatever the U.S. does," Pho added, "China is always in the background."
Economic reasons for ending the ban
Vietnam is strategically located between northeast and southeast Asia with a 2,000-mile coastline on the South China Sea -- a consideration for the U.S. as it repositions military forces in the region.
Twenty-one years after America and Vietnam normalized diplomatic relations, "they really want to take this relationship beyond the past of the Cold War" and a weapons ban that "was a symptom of our war there," Pho said.
There's also a strong economic component behind the decision to lift the ban. The Obama administration has made Asia a centerpiece of its larger foreign policy strategy, seeing the region and its burgeoning middle classes as central to future U.S. economic growth: an enormous market for American exports that can create jobs at home.
Vietnam fits neatly into that vision. It is one of the most pro-American countries in the region, according to a 2015 Pew Survey that found 76 percent of Vietnamese see the U.S. favorably (while fewer than 20 percent saw China favorably). Vietnam is also a party to the Trans Pacific Partnership trade pact that the administration has been championing. The U.S. is the country's seventh-largest investor with trade between the countries worth $44.5 billion last year.
Lifting the weapons ban, put in place to protest human rights abuses, will deepen economic and military ties to Vietnam, defense analysts said, benefiting American weapons manufacturers.
The Obama administration "regards its moves to strengthen engagement with Vietnam as part of its 'rebalancing' strategy," said Jon Grevatt and Paul Burton with IHS Jane's, a global risk consultancy. "A key element of this effort is military sales."
That's a potential boon for U.S. defense contractors. Vietnam is the world's eighth largest weapons importer and is spending increasing amounts on its self-defense, with arms imports rising 699% from 2011 to 2015, according to the Stockholm International Peace Research Institute.
An IHS analysis found that Vietnam needs improved maritime-security capabilities, including maritime patrol aircraft, coastal radars, and naval craft including coastal patrol vessels -- all of which U.S. companies can sell to Hanoi.
And an added benefit is that those sales now won't necessarily go to Russia, a longtime Vietnam supplier.
"With the ban lifted in full, Vietnam is finally shedding Moscow's influence and will be able to purchase land systems and a wider spectrum of military aerospace platforms and systems from the U.S." that will support its efforts to modernize, Grevatt and Burton wrote.