President Biden, fresh from building up his relationship with Prime Minister Modi and other G20 leaders during their parley at New Delhi, will meet Vietnam’s leaders Sunday at Hanoi in what may be the most dramatic stop of his swing through Asia.

Mr. Biden pretty well knew what to say to the “neutral” Mr. Modi, whom he had hosted amid somewhat ambiguous pleasantries at Washington in June. He didn’t have to worry about getting upstaged by Presidents Xi or Putin, who weren’t even there.

It’s Vietnam, where he’s meeting the general secretary of the ruling Communist Party, Nguyen Phu Trong, on Sunday whom he’s really out to impress. He’ll be there nearly 50 years after the Congress of the United States betrayed Vietnam, cutting off military supplies and abandoning them to the Communists. 

The North Vietnamese forces then swept down from the Central Highlands and completed their conquest of “South” Vietnam that we abandoned. It was a debacle far more  breathtaking than that in which Mr. Biden surrendered Afghanistan to the Taliban.

The “North” Vietnamese defeated the South Vietnamese army and totally humiliated the Americans, who’d lost 58,220 KIA — killed in action — and an enormous store of treasure supporting the Saigon government. Now the Americans are coming close to embracing Vietnam — yes, “Communist” Vietnam — as an ally.

In a diplomatic sleight of hand, they’re escalating their relationship from that of “strategic partner” to “comprehensive strategic partner” — a verbal distinction that will impress no one other than those who count, including four others in the same category, notably Communist China and Russia, plus India and South Korea.

“Vietnam has decided to upgrade its relations with the United States because of changes in the global strategic environment,” says an Australian consultant, Carlyle Thayer, who analyzes Asian issues. “Russia has become weakened, isolated, and dependent on  China” while “China has resumed pressure on Vietnam in the South China Sea.”

Mr. Biden is not the first American president to go to Vietnam. A quarter century after the fall of the American-backed Saigon regime in 1975, President Clinton, who had dodged the draft, went to Hanoi in 2000, President George W. Bush in 2006, President Obama in 2016, and President Trump, to Danang in 2017 and Hanoi in 2019. 

This visit, though, may  be the most portentous, moving Washington toward Hanoi in a relationship laden with military, anti-China significance. No, Mr. Biden isn’t likely to make a deal for selling arms to Vietnam. Their upgraded relationship will initially be about high tech, producing semiconductors, even electrical vehicles.

If Communist China keeps chasing the Vietnamese out of the rights for drilling for oil and natural gas in the South China Sea, though, it’s hard to predict how things might look. Might America consider sending arms to Hanoi — albeit not anywhere near the same quantity as the arms we shipped to Vietnam in “the old days.”

Could Vietnam become a de facto partner in the line that America is forming against Chinese expansionism? American aircraft carriers have already visited Vietnam, sending the message that American warplanes could take off from their decks under Vietnamese protection.

President Biden, at the G20, had the chance to promote the influence of American-backed institutions, including the World Bank and International Monetary Fund, as antidotes to China’s Belt and Road Initiative, by which China is extending its influence across South and Southeast Asia to the middle east and even Europe.

Might Vietnam provide a vital link in what the Chinese see as an American effort to encircle, if not “contain,” China? From South Korea to Japan, to the Philippines, to Australia, America has put together alliances that form a bulwark against Chinese expansionism.

No wonder Mr. Biden’s national security adviser, Jake Sullivan, seemed so happy about the visit, which he called “a remarkable step in the strengthening of our diplomatic ties” reflecting “the leading role that Vietnam will play in our growing network of partnerships in the Indo-Pacific.”

It’s likely that Vietnam could have limits to how prepared it is to offend China, considering that China looms large across its northern frontier. It’s still possible that, eventually, America could be aiding Vietnam’s armed forces with specialized weapons intended to blast holes in China’s armor.

China isn’t the only target of Mr. Biden’s foray into Vietnam. Implicit in the bond America is forming with its erstwhile enemy is a message for North Korea — and Russia, too.

The North Koreans have long since made clear their  unhappiness that Vietnam, though a Communist country, has a totally different attitude from what might have been expected in its relationship with their own Communist regime. The fact that Vietnam was once divided between North and South, as is the Korean peninsula, adds to the irony.

North Korea’s leader, Kim Jong-un, is compensating for that disappointment on Saturday with a celebration of the 75th anniversary of the founding of the Democratic People’s Republic of Korea under his grandfather,  Kim Il-sung. 

On parade is likely to be a show of fearsome weaponry, including the Hwasong-18, a solid-fuel intercontinental ballistic missile that’s capable, theoretically, of carrying a nuclear warhead to targets anywhere in North America, notably the 50 American states. North Korea first showed off the Hwasong18 on another 75th anniversary — that of the founding of the Korean People’s Army in February.

It’s even possible the parade will feature a scaled-down model of the new nuclear submarine that the North has just unveiled —  another weapon that’s likely robbed money from the country’s impoverished peasants, on the verge of another famine.

Nor is the parade the only show of North Korean strength. In the  near future, Mr. Kim is expected to meet Russia’s President Putin, probably in the port of Vladivostok, where they met nearly four years ago but maybe, just maybe, in Moscow, where Mr. Kim’s father, Kim Jong-Il, saw Mr. Putin 22 years ago.

Mr. Biden, by going to Hanoi, is showing the possibilities for getting along with a Communist state even as Mr. Kim and Mr. Putin are believed to be on the verge of a major arms deal. North Korea is already producing artillery shells that Russia badly needs for its forces in Ukraine, and there’s every possibility the North will eventually be offering basic infantry weapons, including AK-47 rifles and machine guns.

An analyst in Seoul, talking anonymously, told me that one reason Russia is turning to North Korea for weapons, in violation of UN sanctions, is they’re made a lot more cheaply there than in Russia. Russia in turn could offer technology and high-tech products to North Korea as well as oil and food shipped by rail across its narrow, 11-mile-wide border along the Tumen River and in their territorial waters after the river flows into the sea.

Top American officials, including Mr. Sullivan, and the State Department spokesman, John Kirby, have warned North Korea of dire “consequences” if it makes a deal with Russia on arms. Yet what might the Biden administration be prepared to do? Not much beyond rhetorical flourishes.