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For the foreseeable future, no relationship is more vital to international stability than that between the United States and China. Yet Donald Trump and the Chinese authorities have foolishly introduced dangerous new uncertainties into the equation.

China resolved a potential crisis on Tuesday by returning an underwater research drone that it had seized on Dec. 15 from a United States Navy ship in international waters near the Philippines. Still, the episode stoked fresh tensions and gave Mr. Trump an excuse to double down on his campaign promise to adopt a tougher and less predictable line toward Beijing.

Mr. Trump began this month by taking a postelection phone call from the president of Taiwan, Tsai Ing-wen, and a week later suggested that he might jettison the United States’ longstanding acceptance of a “one-China” policy. It’s unclear what Mr. Trump means by this, or if he understands the threat that a breakdown in relations would pose for Taiwan, an American partner, as well for broader American interests in Asia. Or maybe he understands and just doesn’t care.

Maintaining a delicate balance on Taiwan’s status has been a foundational principle of United States-China relations and crucial to maintaining peace in Asia. Beijing considers Taiwan a renegade region that will eventually be incorporated in China, by force if necessary. Since 1978, the United States has recognized Beijing as China’s sole government, breaking diplomatic ties with Taiwan a year later, but Washington continues to have unofficial ties with Taiwan, sells weapons to the self-governing island, and has hinted it may defend Taiwan if it is attacked.

Mr. Trump told Fox News recently that he might reconsider the one-China policy as a way to exact Chinese concessions in disputes over currency manipulations, trade and Beijing’s moves to claim rocks and reefs in the South China Sea. He may think he is making an opening bid, but for China, Taiwan’s status is nonnegotiable.

Such talk, some experts fear, might mean that Mr. Trump would formally recognize Taiwan or embolden Taiwan to declare independence from China, either of which would bring swift reprisals. Alternatively, some worry that if China offers the right deal, Mr. Trump might abandon Taiwan by ending cooperation and billions of dollars in arms sales. Such a shift would shake the foundations of every international alliance and partnership the United States has spent decades building.

China, the world’s second-largest economy, is America’s largest trading partner, besides being a nuclear power and a veto-wielding member of the United Nations Security Council. Heedless of these concerns, Mr. Trump has threatened to slap 45 percent tariffs on China and start a trade war that would penalize American consumers, who would have to pay more for imported Chinese goods. It would also hurt American businesses that seek to sell their products in the Chinese market. Beijing has already thrown a punch, warning last week that it could place sanctions against General Motors or Ford for monopolistic behavior, possibly as a response to an unfavorable shift in American policy.

There are plenty of ways for China to retaliate, from breaking off diplomatic relations if the United States formally recognizes Taiwan to buying planes from Europe’s Airbus instead of Boeing, to refusing to help curb North Korea’s nuclear program. There could also be more shows of force like the Chinese fighter jets that flew close to Taiwan late last month. And China could also restrict its investments and tourism to Taiwan, which would not be good for the island or the region.

It’s not uncommon for new presidents to put their own stamp on policy, even longstanding policy. But this requires assembling a staff and conducting a full review of the impact of any changes. Mr. Trump has few Asia experts in his circle, and some advisers have urged him to take up a hard line without any sense of long-term repercussions in the region.

President Obama tried to manage a more assertive China by pursuing deeper military cooperation with smaller Asian nations and negotiating the Trans-Pacific Partnership, a 12-nation agreement setting higher standards for trade. Mr. Trump has firmly rejected that deal, with no interest in renegotiating it. His early moves show only his intent to be a disrupter, no matter the consequences to American security.