U.S. President Obama on this week lifted a half-century ban on selling arms to Vietnam, a move that is raising concerns among some Vietnam War veterans.

“At this stage, both sides have established a level of trust and cooperation, including between our militaries, that is reflective of common interests and mutual respect,” said Mr. Obama in Hanoi during his first visit to the country. “This change will ensure that Vietnam has access to the equipment it needs to defend itself and removes a lingering vestige of the Cold War.”

He said every U.S. arms sale would be reviewed case by case to take into account Vietnam’s human-rights record.

At a joint press conference with Mr. Obama, Vietnamese President Tran Dai Quang said the two countries are “former enemies turned friends.”

“And now we are comprehensive partners,” he said.

More than 58,200 U.S. soldiers were killed in Vietnam before the fall of Saigon in 1975. But the administration saw advantages in easing the embargo, both as a warning to expansionist China and as leverage to compel the communist regime in Hanoi to improve its record on human rights.

“We are thinking through how our evolving security cooperation [is] going to look moving forward,” said White House Deputy National Security Adviser Ben Rhodes prior to the visit. “They regularly raise this issue with us. We are looking at, of course, how our broader relationship is evolving, including our continued commitment to support human rights in Vietnam.”

The notion of selling more lethal weaponry to Vietnam is a bitter irony to some veterans of the war and to some Vietnamese-Americans who fled the communists.

“They are still a communist country,” said retired Army Maj. Wulf Linden, a Georgia resident who served two tours in Vietnam. “If it’s the cream of our crop [of weapons systems], obviously that would be a bad mistake. It would be nice if we’re not selling to a regime that’s a documented communist regime. That’s my concern.”

Army veteran Ralph Riccio, a Pennsylvania resident who served in Vietnam in 1968 and 1969, said he has mixed feelings about the potential arms sales.

“As somebody who got shot at in Vietnam, I find it a little bit ironic that we’re selling them weapons now,” Mr. Riccio said. “I find it a little bit uncomfortable to come to grips with.”

But he noted other examples of the U.S. embracing former wartime adversaries, such as Germany and Japan after World War II.

“I understand that one day somebody’s an enemy and the next day they’re not that much of an enemy,” he said.

Joe Frank, a former national commander of the American Legion and an Army veteran who lost the use of his legs to a land mine in Vietnam, said he never dreamed in the late 1960s that the U.S. would one day seek a closer alliance with Hanoi. But he said he accepts the politics of the move.

“I lost a lot of friends in Vietnam. They’re on that black granite wall [the Vietnam Veterans Memorial in Washington],” Mr. Frank said. “A soldier like me, just like the Vietnamese, we were just taking our orders from those up above. This [arms sales] is politics. It doesn’t bother me as long as long as this is well-researched.”

Beefing up Vietnam’s defenses is a crucial step for the Obama administration in standing up to China’s military influence, said James Zumwalt, a retired Marine infantry officer who served in Vietnam and has written extensively about Southeast Asia.

“I am not an Obama fan,” Mr. Zumwalt said. “But this is one of the few things they’ve done in the foreign policy area that I totally agree with.”

He said the administration must do more to confront China over its territorial claims in the South China Sea, a region of major shipping lanes and energy deposits where Beijing has been building artificial islands.

“China will try to do anything and everything they can in the remaining 200-some days that Obama’s in office to establish themselves in the South China Sea,” Mr. Zumwalt said. “That’s why I think it is absolutely imperative that Obama show some backbone and take this step and do it in a way that leaves no doubt in the minds of the Chinese that we’re upset with what they’ve done. We’ve let China get away with so much, this is kind of an inexpensive way of conveying a message to China.”

Mr. Obama said his decision to lift the ban on arms sales “was not based on China,” although he added that he wants to help Vietnam “improve their maritime security posture.”

“My decision to lift the ban really was more reflective of the changing nature of the relationship,” the president said Monday. “Given all the work we do together across the spectrum of economic, trade, security and humanitarian efforts, that it was appropriate for us not to have a blanket across-the-board ban.”

Vietnam has purchased much of its military equipment, including submarines, from Russia and is seeking costly high-tech weaponry from the U.S.

The U.S., however, wouldn’t feel the effects of an arms sales agreement overnight, said Richard Fontaine, a defense analyst at the Center for New American Security who served as an adviser to Sen. John McCain, Arizona Republican.

Mr. Fontaine said the anti-Vietnam sentiment in Congress has faded.

“It’s no longer the political obstacle that it was even 10 or 15 years ago,” he said. “The general sentiment on Capitol Hill has been, as long as Vietnam is moving in the right direction — even rather gradually toward opening up the political space and the religious freedom space and the rule of law — then we want to have an increasingly close relationship. At the same time, [on] the defense side of things, I don’t think that you will see and you shouldn’t see the United States sell truncheons and tear gas and the kinds of things that could be used for domestic repression.”

Many Vietnamese-Americans say the repressive communist government doesn’t deserve the benefits of closer military and economic ties with the U.S.

Bao Nguyen, the mayor of Garden Grove, California, has written a letter to Mr. Obama and to Secretary of State John F. Kerry about Vietnam’s “atrocious” human rights record, including its imprisonment of dissidents in solitary confinement without charges.

“While we have an arms surplus to deal with, I don’t think dumping those arms on developing nations is the most effective way to counter China,” Mr. Nguyen said. “That’s because there’s no way to know that the government won’t use those weapons against its own people. Especially a government as corrupt and autocratic as Vietnam.”

Mr. Zumwalt said the U.S. sometimes needs to work with unsavory partners, as it did with autocratic Filipino President Ferdinand Marcos during the Vietnam War.

“Would I like to see a democratic government in Vietnam? I sure would,” Mr. Zumwalt said. “But I see Vietnam as being the lesser of two evils. We’ve got to show some backbone. We’ve got to show it with the Vietnamese, and we’ve got to convey the point to China that enough’s enough.”

Mr. Obama’s 47th foreign trip will include three days in Vietnam and a Group of Seven summit in Japan, where he will become the first U.S. president to visit Hiroshima.

He will promote the 12-nation Trans-Pacific Partnership trade agreement, which includes the U.S., Vietnam and Japan. The deal is stalled in Congress and faces strong opposition from the leading presidential candidates and most Democrats, who say it doesn’t protect U.S. workers from unfair trade practices.

On the issue of human rights in Vietnam, five Republican senators sent a letter Friday to Mr. Obama calling Hanoi “one of the most repressive regimes in the world” and urging Mr. Obama to press Vietnamese leaders to do more to improve religious freedom and other human rights. The letter was authored by Sens. Marco Rubio of Florida, John Boozman of Arkansas, John Cornyn of Texas, James Lankford of Oklahoma and Bill Cassidy of Louisiana.

Human rights groups are urging Mr. Obama to call on Vietnamese leaders, who held a rubber-stamp election of their one-party rule last weekend, to release political prisoners and other activists.

“Obama should stand next to Vietnam’s leaders in public and call on them to respect the right to freely choose government representatives, stand for office and peacefully advocate for democracy,” said Brad Adams, Asia director of Human Rights Watch. “If this trip is partially about legacy-building, as some suggest, there can be no more meaningful legacy than helping the people of Vietnam achieve fundamental reforms.”