On the eve of his anointment by the Chinese Communist Party for a second five-year term as China’s leader, Xi Jinping seems to be the master of all he surveys. He has centralized economic and national-security policy in his office. The military and the police are firmly under control. Legions of corrupt officials—some of them political rivals, others caught brazenly on the take—are in jail. Dissidents and activists have been sidelined or locked up.

But Mr. Xi’s most consequential political battle today has received little attention. As he has tightened his grip on the party and the military, he now faces only one potential set of genuine rivals: China’s new class of wealthy entrepreneurs. China’s communist leaders have long been known to dread a Soviet-style collapse, but Mr. Xi and his senior colleagues are equally worried about replicating what came in its aftermath: the rise of the Russian oligarchs, who snatched control of state assets and turned themselves into billionaires and pushy political players. Mr. Xi is determined not to let that happen in China.

Some of China’s new tycoons have challenged the state; others have acquiesced after clashes with the authorities; most have kept their heads down and concentrated on making money. But whether or not the entrepreneurs are taking on Mr. Xi, Mr. Xi has decided to pre-empt any threat by taking the fight to them first.

Mr. Xi’s campaign to corral the private sector seems to have begun in earnest in June, with the disappearance of the swashbuckling businessman who had become a standard-bearer for local tycoons leading a new wave of aggressive deal-making overseas. Wu Xiaohui, the chairman of Anbang Insurance Group, went the way that communist members who fall afoul of the system do, vanishing without official explanation into the party’s extralegal detention system.

Only months earlier, Mr. Wu had been leading negotiations to spend $14 billion buying Starwood Hotels & Resorts Worldwide Inc. before the deal collapsed. Mr. Wu hasn’t been heard from since his detention; no charges have been filed.