President Trump’s recently concluded trip to Asia had the potential to advance important American security and economic interests. Played correctly, his ambitious five-country, 12-day trip could have steadied his administration’s rocky start in this vital region. Instead, it left the United States more isolated and in retreat, handing leadership of the newly christened “Indo-Pacific” to China on a silver platter.

The trip began with solid performances in Japan and Korea, where Mr. Trump’s relatively measured words left key allies reassured of the United States’ commitment to their security. The president largely shelved his belligerent trade rhetoric, called for allies to buy more American military hardware and reopened the door to diplomacy with North Korea. Weather curtailed his surprise trip to the Korean Demilitarized Zone, but that may have been a blessing, since hostile words might have prompted hostile action.

But in China, the wheels began to come off his diplomatic bus. The Chinese leadership played President Trump like a fiddle, catering to his insatiable ego and substituting pomp and circumstance for substance.

China always prefers to couch state visits in ceremony rather than compromise on policy. This approach seemed to suit President Trump just fine, as he welcomed a rote recitation of China’s longstanding rejection of a nuclear North Korea and failed to extract new concessions or promises. He also settled for the announcement of $250 billion in trade and investment agreements, many of which are nonbinding and, in the words of Secretary of State Rex Tillerson, “pretty small.” Missing were firm deals to improve market access or reduce technology-sharing requirements for American companies seeking to do business in China.