“China is now capable of controlling the South China Sea in all scenarios short of war with the United States,” the new commander of the United States Indo-Pacific Command, Adm. Philip S. Davidson, acknowledged in written remarks submitted during his Senate confirmation process in March.

He described China as a “peer competitor” gaining on the United States not by matching its forces weapon by weapon but by building critical “asymmetrical capabilities,” including with anti-ship missiles and in submarine warfare. “There is no guarantee that the United States would win a future conflict with China,” he concluded.

Last year, the Chinese Navy became the world’s largest, with more warships and submarines than the United States, and it continues to build new ships at a stunning rate. Though the American fleet remains superior qualitatively, it is spread much thinner.

“The task of building a powerful navy has never been as urgent as it is today,” President Xi Jinping declared in April as he presided over a naval procession off the southern Chinese island of Hainan that opened exercises involving 48 ships and submarines. The Ministry of National Defense said they were the largest since the People’s Republic of China was founded in 1949.

Even as the United States wages a trade war against China, Chinese warships and aircraft have picked up the pace of operations in the waters off Japan, Taiwan, and the islands, shoals and reefs it has claimed in the South China Sea over the objections of Vietnam and the Philippines.

When two American warships — the Higgins, a destroyer, and the Antietam, a cruiser — sailed within a few miles of disputed islands in the Paracels in May, Chinese vessels rushed to challenge what Beijing later denounced as “a provocative act.” China did the same to three Australian ships passing through the South China Sea in April.

Only three years ago, Mr. Xi stood beside President Barack Obama in the Rose Garden and promised not to militarize artificial islands it has built farther south in the Spratlys archipelago. Chinese officials have since acknowledged deploying missiles there, but argue that they are necessary because of American “incursions” in Chinese waters.