They have poetic names – Mischief Reef, Triton Island, Fiery Cross Reef. Just a few years ago they were dots of coral in the middle of the South China Sea, baptized by ancient navigators. Today they are some of China’s most strategic military bases, dredged into artificial existence and bristling with missile systems, runways, and radar defenses. China’s claim to own most of the South China Sea has been ruled illegal by an international court. President Xi Jinping promised Washington he would never deploy military assets to the new islands. Neither has made any difference to Beijing’s drive to impose its strategic grip on a waterway that carries 20 percent of world trade. “China is now capable of controlling the South China Sea in all scenarios short of war with the United States,” according to the new head of US Indo-Pacific Command. And that is just one step toward a more ambitious goal, most observers say: to supplant the US as the preeminent power in Asia and the western Pacific.

Five years after Beijing began reshaping the South China Sea to suit its interests in the teeth of international opposition, China has all but taken control of the vital waterway.

China’s success in asserting an unbreakable strategic grip on the South China Sea through the creation of militarized artificial islands brings it another step closer to its ultimate goal: supplanting the United States as the preeminent power in Asia and the Pacific.

That goal may still be a distant one, many analysts say; the US Navy is still much larger than China’s, to say nothing of Washington’s network of alliances in the region that took decades to develop.

But a recent report by the Lowy Institute, a think tank in Sydney, suggests that an increasingly assertive Beijing is rapidly closing in on Washington in the two giants’ regional power struggle. And nowhere is that more evident than in the South China Sea, the corridor for over 20 percent of world trade, which even US military officials concede is now firmly in Beijing’s hands.

“The ability of the United States to constrain China's expansion in the South China Sea has been close to nothing,” says Robert Ross, an expert in Chinese defense policy at Boston College. “It's far from clear how it can do anything more robust than it is doing” already.

Since 2013 China has assiduously sought to change the “facts in the water” in the South China Sea, taking strategic charge of the region while avoiding direct military conflict with other countries pursuing competing territorial claims and, most importantly, with the United States.

In 2015, during his first state visit to the United States, Chinese President Xi Jinping promised not to militarize the islands, a pledge he has since broken.

Mine, all mine

At first, China focused on transforming various reefs and islets into artificial islands and claiming them as its own, in defiance of international law. Then the People’s Liberation Army surreptitiously started to militarize the islands, equipping them with airstrips, radar and missile systems.

In 2016, when an international tribunal found that China’s expansive claim to sovereignty over the South China Sea had no legal basis, Beijing simply ignored the ruling and went on building islands.

Now China appears poised to go even further.

In April, Mr. Xi oversaw the largest naval parade in China’s history after two days of military drills in the South China Sea. Last month, China landed long-range bombers for the first time at an airfield built on one of its man-made islands. Recent satellite images show newly constructed buildings that could soon be home to China’s first troops based in the hotly contested Spratly archipelago, where Taiwan, Malaysia, the Philippines, and Vietnam all maintain territorial claims.

When the man-made islands are occupied, “China will be able to extend its influence thousands of miles to the south and project power deep into Oceania,” US Adm. Philip Davidson wrote in testimony to Congress in April. “China is now capable of controlling the South China Sea in all scenarios short of war with the United States.”

During a ceremony at Pearl Harbor on Wednesday, Davidson became head of the US Pacific Command, which was renamed the US Indo-Pacific Command to underscore the growing importance of India to the Pentagon.

Davidson is taking over at a time of heightened tensions between China and the US over the South China Sea. Last week, Washington withdrew an invitation for Beijing to participate in multinational naval exercises later this summer in what it called “an initial response” to China's militarization of the waterway.

Then, on Sunday, the US conducted its latest “freedom of navigation” patrol, sending two navy destroyers within 12 miles of islands in the Paracel group to challenge Beijing’s territorial claims there. Earlier this week, US Defense Secretary Jim Mattis said American ships would maintain a “steady drumbeat” of naval operations around the Spratly Islands.

Davidson’s predecessor, Adm. Harry Harris, said at the Pearl Harbor ceremony that while North Korea is the most imminent threat to the United States, China remains Washington’s biggest long-term challenge.

“Without focused involvement and engagement by the United States and our allies and partners,” Harris said, “China will realize its dream of hegemony in Asia.”

High-stakes arm wrestling

Although Xi has assured the world he won’t seek hegemony, he has expressed his belief that the vast majority of the South China Sea has been “China’s territory since ancient times.” In a speech he gave last October, Xi highlighted “steady progress” in the construction of new islands as a major achievement of his first term as president.

Vietnam, the Philippines, Malaysia, Taiwan, and Brunei all have overlapping territorial claims in the South China Sea and are wary of China’s intentions. On Wednesday, the Philippines warned that it was prepared to go to war if its troops on bases in the South China Sea were harmed.

While the risk of a military confrontation may at times seem high, Zhang Baohui, a professor of international relations at Lingnan University in Hong Kong, says the actions both sides have taken so far show that neither wants to start a war.

“The South China Sea has become a battleground over prestige in the region,” Prof. Zhang says. “China wants to prove that its rise cannot be stopped by the US. The US wants to show the region that it’s still in charge.”

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How much longer the US will remain the leading power in Asia is an open question. With a defense budget nearly four times the size of China’s, and 11 aircraft carriers to China’s two, the US is likely to retain the upper hand militarily for years to come. But Zhang says that the contest for regional primacy is about much more than military strength.

“Military is really only part of the competition. China’s game is to win primacy through its economic power,” he argues, adding that China’s economic influence in Asia far outweighs that of the United States, and is growing. “In the long-term, China establishing some kind of primacy is inevitable.”