When President Obama declared in 2011 that he wanted U.S. foreign policy to pivot to Asia, some derided the move as a clumsy attempt to flee the messy conflicts of the Middle East.
But the pivot has actually worked pretty well – as will be evident when Obama travels to Asia this week. Almost every country in the region is clamoring for a closer relationship with the United States.
The most striking case is Vietnam, most of whose leaders are old enough to have fought in their country's war with the United States. The communist regime has been openly courting a deeper military relationship, and has even invited the U.S. Navy to return to Cam Ranh Bay, its base during much of the war. During his visit, Obama is expected to announce an expansion of American military sales.
The impetus for this rapprochement is China, Asia's increasingly assertive great power. Beijing's pursuit of sovereignty over the islands of the South China Sea, most of which are also claimed by other countries, has flung China's neighbors into the arms of the United States.
"Any time China tries to put its thumb on any of its neighbors, that makes them enthusiastic about getting close to us," noted Derek Chollet, a former Defense Department official.
Only a few hundred miles from Vietnam's coast, Chinese construction teams have been dredging the seafloor and using landfill techniques to increase the size of China's territories, then building infrastructure to support military facilities.
The newly-built islands aren't much use in a military conflict with the United States; U.S. Navy officers dismiss them as sitting ducks. But as military bases, they could still help Beijing intimidate weaker neighbors such as Vietnam and the Philippines.
Eventually, the islands could also enable China to assert economic rights to the estimated 11 billion barrels of oil beneath the seabed. Even fishing rights are at stake; China's fishing industry, the world's largest, employs more than 14 million people.
On a visit to Washington last year, Chinese President Xi Jinping promised not to "militarize" the islands, but he never defined what the term meant. Some Chinese officials later said Xi's policy merely banned "major offensive weapons." That created alarm in the Pentagon and prompted the Obama administration to sharpen its denunciations of the construction projects.
This is an asymmetric struggle; there aren't many practical steps the United States can take to stop China's dredging. The Pentagon sends ships near the islands to assert U.S. freedom of navigation, but that hasn't slowed the construction.
"It's not clear what else we can do," a former official told me. "We're not going to start a war, and we're not going to occupy an island ourselves."
The United States does have one asymmetric advantage of its own: its ability to forge stronger alliances with China's worried neighbors – not only Vietnam, but the Philippines, Malaysia and others as well.
A stronger Vietnamese navy – one that holds joint maneuvers with the U.S. Navy – would deny China some of the military advantage it hoped to gain from building all those airstrips.
The idea, in short, is to raise the long-term cost to Beijing. Of course, that strategy works only if the United States is willing to invest in those stronger relationships – through not only a U.S. military presence, but expanded trade agreements, too.
So Obama faces what Chollet calls a "reassurance challenge."
"All these countries are looking for reassurance that the United States will be there," he said. "They all want the United States to do more – and we can't possibly deliver everything they want."
Indeed, all three remaining candidates in the presidential campaign have been critical of the Trans Pacific Partnership, Obama's trade agreement with most of Asia except China. Donald Trump, in particular, has promised to scrap TPP if he's elected.
That would be a particularly acute problem for Vietnam, a low-income country which would be a major beneficiary of the agreement. Administration officials warn that if Congress refused to ratify TPP, Vietnam and other developing countries will have little choice but to tie their economies more closely with China's.
In other words, if Trump gets his way, the biggest beneficiary in Southeast Asia might well be China.