THERE IS reason for concern that President Trump, who will begin a two-day summit Thursday with Chinese President Xi Jinping, is not prepared to inaugurate what will probably be the most important foreign relationship of his presidency. Mr. Xi will arrive with a polished list of Chinese demands; Mr. Trump, who has yet to staff most of the senior Asia positions in his administration or conduct a review of China policy, appears to be improvising. The danger is that he, like Secretary of State Rex Tillerson during his recent visit to Beijing, could be drawn into China’s conception of a “new form of great power relations” before he has had the chance to formulate one of his own.

The confusion about administration policy is partly the result of a positive development: Mr. Trump has retreated from some of the counterproductive China-bashing he promoted during the presidential campaign and immediately afterward. He did not designate the Xi regime as a currency manipulator on “day one” of his presidency; he told the Financial Times this week that he would postpone any talk of punitive tariffs on Chinese goods. He assured Mr. Xi during their first substantial phone call that he supported the one-China policy, reversing a hint that he might not.

These necessary adjustments have been accompanied by less explicable retreats. Since Mr. Trump took office, U.S. naval forces reportedly have not received administration permission to challenge Chinese claims in the South China Sea through “freedom of navigation” exercises. And Mr. Trump handed Beijing a major geopolitical coup by withdrawing from the Trans-Pacific Partnership, a step that left Mr. Xi with the opportunity to set the economic rules for the region.

If the administration has focused on an issue for the summit, it would appear to be North Korea, which a White House briefer says is “a matter of urgent interest for the president.” Mr. Trump appears to be betting that he can, unlike the three presidents before him, persuade the Chinese leadership to bring serious pressure to bear on the Pyongyang regime, which is racing to develop the capability to strike the United States with nuclear-armed intercontinental missiles.

The president is right to push Mr. Xi on the North Korean threat; he should make clear that the United States will proceed with the deployment of the THAAD antimissile system in South Korea in spite of China’s objections. But it is not certain that Mr. Trump can deliver on his threat that “if China is not going to solve North Korea, we will.” Rather than tempt Mr. Xi to call his bluff, Mr. Trump should be offering assurances that change on the Korean Peninsula can be managed in a way that protects Beijing’s legitimate interests. The stick Washington can credibly wield is not the threat of unilateral military action, but steps to punish Chinese banks and companies that fail to observe U.N. sanctions.

Administration officials suggest the summit discussions will span a variety of issues; one of these should be human rights. Mr. Xi has presided over the most aggressive crackdown on dissent in China in decades, and he is in the midst of stifling Hong Kong’s relative freedom. By raising these issues, Mr. Trump can not only side with those in China who believe in democratic values, he can also make clear to Mr. Xi that their “great power relations” will not be exclusively on Beijing’s terms.