Mr. Trump arrives in Asia as a wounded leader, with low ratings at home and a stalled legislative agenda, and dogged by revelations of Russian interference in the 2016 election on his behalf that have led to indictments against former advisers. His furious denunciations of the constitutional constraints he faces in trying to scuttle the investigation bring to mind the authoritarian leaders of whom he is so fond. This, along with his disregard for human rights, further diminishes the democracy that is America’s greatest source of strength and influence in the region.

A course correction the president made on Friday suggests some awareness about what’s at stake on this trip. Departing Washington, he announced he would reverse his earlier decision and attend the East Asian summit in the Philippines, the main forum where leaders of the United States, China, Russia and other Asian nations discuss tensions like those over the South China Sea and transnational threats like terrorism. If Mr. Trump were a no-show, it would intensify doubts about America’s regional commitment.

To address these doubts, Mr. Trump is expected to use a speech at the Asia-Pacific Economic Cooperation meeting in Vietnam to call for a “free and open” Indo-Pacific region, with the goal of containing China, where Mr. Trump will have a state visit with President Xi Jinping. Secretary of State Rex Tillerson has raised the possibility of countering Mr. Xi’s $1 trillion “one belt, one road” initiative of roads, pipelines and other projects linking China to Europe and Asia with “alternative financing measures, financing structures.” But where the money might come from is a major question.

In contrast to Mr. Trump’s weakness, Mr. Xi just closed a Chinese Communist Party congress at which he was elevated to the same exalted status as the nation’s founding father, Mao Zedong, by writing his name and ideas into the party Constitution. Mr. Xi was already a powerful, authoritarian leader expanding China’s reach militarily and politically, as well as economically, and he seems to have a clearer vision of where he is taking his country and how to get there, even if some of the means may be appalling.

Mr. Trump has already undercut his two main priorities for the trip. He wants China to exert more pressure on North Korea to end the North’s nuclear weapons program by completely cutting off purchases of North Korean coal, closing North Korean bank accounts and sending North Korean workers home. Mr. Xi has repeatedly rejected going as far as Mr. Trump demands, because doing so could destabilize North Korea, and he has pushed for a negotiated solution to the crisis. But Mr. Trump, reversing his own secretary of state, has refused talks, preferring bombastic threats to “destroy” North Korea that will be magnified if he repeats them in Asia.