Last month, the USS Carl Vinson became the first American aircraft carrier to visit Vietnam since the Vietnam War ended in 1975. Coming alongside the news that a record 23 nations from Southeast Asia and beyond would be joining biennial naval exercises in the eastern Indian Ocean, it was a potent reminder of just how eager the nations surrounding the South China Sea are to embrace powerful allies to fend off a rising China.

But as Beijing’s regional clout continues to grow, it can be hard for weaker nations to resist it, even with these allies’ support. Barely three weeks after the Carl Vinson’s visit, the Vietnamese government bowed to Chinese pressure and canceled a major oil drilling project in disputed South China waters.

It was yet another sign of the region’s rapidly shifting dynamics. For the last decade, the United States and its Asian allies have been significantly bolstering their military activities in the region with the explicit aim of pushing back against China. But Beijing’s strength and dominance, along with its diplomatic, economic and military reach, continue to grow dramatically.

In recent days, Beijing has conducted major military drills, including reportedly sending its aircraft carrier through the Taiwan Strait and flying jets between Japan’s southernmost islands (the latter of which was explicitly described by the Chinese military as a preparation for future wars). These drills are both a response to recent U.S. military actions and a bold statement of China’s intent to keep up the pressure.

Western military strategists worry that China will, in time, be able to block any activity in the region by the U.S. and its allies. Already, satellite photos show China installing sophisticated weapons on a range of newly reclaimed islands where international law says they simply should not be present. In any war, these and other new weapons that China is acquiring could make it all but impossible for the U.S. Navy and other potential enemies of China to operate in the area at all.

A simple issue lies at the heart of the problem — Beijing’s claim to sovereignty over almost the entire South China Sea basin. That assertion — first made by the exiled Republic of China government in Taiwan in 1947 and then repeated by its mainland communist counterpart since — was flatly rejected by a United Nations tribunal in 2016. But that hasn’t stopped Beijing from doing more and more to secure its position in the area.

China’s increasing confidence in asserting control over the South China Sea has clearly alarmed its neighbors, particularly the Philippines, Vietnam, Malaysia, Indonesia and Brunei, all of whom have competing territorial claims over waters that China claims for itself. But it also represents a major and quite deliberate challenge to the U.S., which, as an ally to all these nations, has essentially staked its own credibility on the issue.

Over the last several years, it has become common practice for U.S. warships to sail through nearby waters, pointedly refusing to acknowledge Chinese demands that they register with its unilaterally declared air and maritime “identification zones” (which the U.S. and its allies do not recognize). The last such voyage — dubbed a “freedom of navigation” operation or, in naval parlance, a “FONOP” — took place March 23, when the destroyer Mustin reportedly passed within 12 nautical miles of Mischief Reef in the Spratly Islands, which is claimed by both China and the Philippines.

The Mischief Reef FONOP fits a much wider pattern of increasingly assertive naval activities by Washington and its allies in the region. Earlier in March, the U.S. announced the first deployment of its cutting-edge F-35B vertical takeoff Joint Strike Fighter — by far the most sophisticated aircraft of its type — to an amphibious assault ship in the region.

This kind of military tit for tat appears to be escalating; China now says it plans monthly military exercises in the South China Sea, a substantial increase. That will likely be met by intensified American and allied activity: A British warship is shortly scheduled to conduct its own FONOP, and the U.S. has pushed Australia to join the program (although whether it will do so remains a topic of considerable debate within the country).

None of this, however, addresses the seismic regional change produced by China’s island-building strategy. Over the last five years, China has built ever more sophisticated military and industrial outposts on disputed atolls and reefs, in some cases transforming scrubby patches of rock that barely broke the surface into large concrete installations.

Once a tiny fishing station on stilts, Mischief Reef is now a well-armed Chinese military base. And China’s land-reclamation efforts continue, aided by a giant new dredging vessel that its Chinese designers have touted as a “magical island-building machine.” Filipino officials say they believe it is only a matter of time before Beijing makes similar moves at Scarborough Shoal, within striking distance of major Filipino population centers and military facilities.

If and when that happens, the Philippines, along with the U.S., will face a difficult decision about how best to respond. At stake here are more than energy resources — although the estimated 11 billion barrels of oil and nearly 200 trillion cubic feet of natural gas that the South China Sea is believed to contain are clearly a factor in the conflict.

But more broadly, China sees this confrontation as a test case for its ability to impose its will on the wider region — and so far it is winning. (At the very least, foreign energy firms are now unlikely to bid to drill in disputed waters; Spanish oil firm Repsol is said to have lost up to $200 million after Chinese pressure prompted Vietnam to cancel its planned exploration.)

The U.S. remains the world’s pre-eminent military superpower, and there is little doubt it could win a fight with China almost anywhere else in the world. In its own backyard, however, Beijing is making it increasingly clear that it calls the shots. And for now, there is little sign anyone in Washington — or anywhere else — has the appetite to seriously challenge that assumption.

Peter Apps is Reuters global affairs columnist.