– President Obama and his Vietnamese counterpart announced new cooperation Monday to bolster both countries' military might, an accord prompted by the rise of China and that until recently seemed inconceivable for the two wartime foes.

Half a century after the U.S. banned the sale of arms to its enemy in the Vietnam War, Obama surprised President Tran Dai Quang with news that he would lift the entire embargo. Vietnam responded with a promise of more sweeping access to its strategically valuable ports for the U.S. Navy.

Human rights advocates were stunned that Obama would take such a step without first exacting pledges that Vietnam's Communist regime will improve its record on human rights.

But Obama said that while he will keep pushing for human rights reform, his decision to strip away the weapons sales ban serves a greater good in giving Vietnam more heft against neighbors who "throw their weight around," an unsubtle reference to China.

The decision remained cloaked in secrecy until Obama's first full day in Vietnam's capital, as did his administration's assessment of another surprise move — an airstrike that killed the leader of the Taliban, who U.S. officials said was blocking peace negotiations in Afghanistan. Obama said he ordered the strike in the interest of helping Afghanistan "secure its own country."

More trade and enterprise

He is pitching increased trade and enterprise in Vietnam and Japan this week as he tries to rally support in Congress for a Pacific trade deal. The pact would open up markets such as Vietnam, with its rapidly expanding economy and growing new middle class, more broadly to U.S. businesses. As an exhibit for his audience back home, Obama also announced new deals for Boeing and General Electric on Monday.

Vietnam, with the fastest-growing economy in Southeast Asia and a strategic position on the South China Sea, has been attractive to the U.S. for some time as both a commercial and military partner. President Bill Clinton helped nudge U.S. public opinion toward a postwar view of the country when he visited Vietnam and eased trade restrictions in the 1990s.

Two years ago, Obama moved to drop part of the arms ban by allowing sales that would boost Vietnam's maritime surveillance and security capability. Among other changes, his decision allowed American companies to sell boats with machine gun mounts to Hanoi.

In the meantime, his administration has continued talking with leaders in Vietnam, a government in which the general secretary of the Communist Party is as influential as the president or prime minister.

In the run-up to the president's visit, emissaries took care not to suggest a quid pro quo for any of the offerings up for discussion. Obama didn't want to make the access to ports contingent on a lift of the ban. Likewise, Vietnamese officials wanted the human rights question separate from military talks.

But as Obama's departure for Hanoi neared on Saturday, one presidential adviser said talks were "trending toward" a series of agreements, all as part of a larger consensus that a stronger, closer friendship would be good for both sides.

Human rights advocates dismissed that logic.

"President Obama just gave Vietnam a reward that they don't deserve," said John Sifton, Asia policy director at Human Rights Watch, saying the U.S. has for years demanded human rights improvements from Vietnam in exchange for closer military or economic ties.

Administration officials haven't ruled out the possibility that Vietnam will free some political prisoners and widen the latitude of journalists, bloggers and dissidents to speak without fear of ­retribution.

Those reforms should come because they're the right thing to do, Obama said Monday. The U.S. doesn't "seek to impose our form of government on Vietnam," Obama said, but will "continue to speak out on behalf of human rights we believe are universal."

Obama also argues that the Vietnamese are taking steps to improve working conditions by embracing the labor provisions in his Trans-Pacific Partnership deal.

Quang, while promising greater access to the Vietnamese ports, did not offer specifics about access to Cam Ranh Bay, the valuable port that served as a U.S. supply point during the Vietnam War.