Chinese J-15 fighter jets launch from the deck of the Liaoning aircraft carrier during military drills in the South China Sea in 2017. (STR/AFP/Getty Images)

EVEN AS China’s trade negotiators were successfully rebuffing the Trump administration’s demands last week, the People’s Liberation Army Air Force made an announcement: A number of bombers, including a heavy long-range model, had landed on an island in the South China Sea. It was another significant step in Beijing’s militarization of disputed territories in the region — and more evidence of how President Trump has been strategically outplayed by his “friend” Xi Jinping.

For years, Mr. Xi’s regime has been steadily building up islets it controls in the Paracel and Spratly islands, even though an international tribunal rejected its sweeping claim to them. Mr. Xi promised President Barack Obama in 2015 that the territories would never be militarized — and then proceeded to do just that. Anti-ship cruise missiles were deployed on three islets in the Spratlys, CNBC reported this month. Previously, radar and communications- ­jamming equipment were installed on both island chains.

The planes reportedly visited Woody Island, Beijing’s largest base in the Paracels, but experts predicted they would soon be spotted on the long runways constructed in the Spratlys. From there, the long-range H-6K bomber could cover much of the South China Sea — through which up to one-third of global trade passes — as well as Southeast Asia and even northern Australia.

In short, while Mr. Trump has been ineptly seeking an unfeasible reduction in the U.S. merchandise deficit with China, the regime has quietly stolen a march in another theater. Even before the landing was announced, Adm. Philip Davidson, the incoming chief of U.S. Pacific Command, told Congress that “China is now capable of controlling the South China Sea in all scenarios short of war with the United States.” The American answer has been limited to verbal protests by the Pentagon and White House, and showy but equally ineffective “freedom of navigation” patrols by U.S. warships past some of the islands. When the deployment of missiles in the Spratlys was reported, White House spokeswoman Sarah Huckabee Sanders promised “consequences.” Other than disinviting China from an upcoming Pacific naval exercise, there have been none.

China’s de facto takeover of islets claimed by several other countries, including U.S. ally the Philippines, and its construction of a military capacity to control vital international sea lanes represent a failure of more than one administration. Mr. Obama also proved feckless in the face of Beijing’s salami-slicing strategy of gradually building up its position; each individual step was judged insufficient to justify a major U.S. reaction. Mr. Trump, preoccupied with obtaining favors on trade and North Korea from Mr. Xi, has been similarly straitjacketed.

But the long-term impact of China’s buildup could be considerably more important than the threat from impoverished and isolated North Korea, much less the trade balance. Countries surrounding the South China Sea could eventually find themselves forced to accept Chinese hegemony in the region. As Mr. Davidson suggested, the United States would be unable to reverse that dominion short of a nearly unthinkable war. In all, it seems like a more worthy problem for presidential attention and strategic planning than exports of soybeans and automobiles.