RUTHFULLY YOURS

Mr. Trump mentioned sovereignty 21 times in that speech. Why? Everyone knew America was a sovereign state, one of nearly 200 in the world. The idea of sovereignty has been firmly established for more than three centuries. Mr. Trump’s defense of it seemed unnecessary.

Yet for more than a decade, President Xi has been dropping audacious hints that China is the world’s only sovereign state. As a result, I have come to believe that Mr. Trump’s defense of sovereignty is essential to maintaining international peace and stability.

The world is full of “experts” who will tell you China and the U.S. are locked in a contest for dominance. Technically, that’s true. The idea that the two nations are struggling for control, however, falsely implies that America is jealously guarding its position atop the international system. That’s Beijing’s narrative. Chinese leaders disparage the U.S. by implying it is in terminal decline and accusing it of attempting to prevent China’s legitimate rise.

In reality, America is preserving more than its role in the international system. It is trying to preserve the system itself—which Mr. Xi is working to overthrow by promoting imperial-era Chinese concepts.

The idea that underpinned the imperial tributary system was that states near and far were obligated to acknowledge Chinese rule. Chinese emperors claimed they had the Mandate of Heaven over tianxia, or “All Under Heaven.”

China repudiated tianxia in the first half of the 20th century and played it down in the second half. But in the 21st century it is making a comeback. “Tianxia is a long Chinese political tradition of practice and ideal that is being revitalized and re-energized in today’s People’s Republic,” Fei-Ling Wang, author of “The China Order: Centralia, World Empire and the Nature of Chinese Power,” told me last week. “The Chinese dream of tianxia, or the China Order, assumes a hierarchical world empire system.”

Mr. Xi’s signature concept is the “Chinese Dream,” or “the great rejuvenation of the Chinese nation.” “Rejuvenation” evokes restoration of the imperial system, and echoes of tianxia could be heard in the slogan for the 2008 Beijing Summer Olympics, which Mr. Xi managed as a member of the Communist Party’s Politburo Standing Committee. “One World, One Dream,” at least to scholars employed by the party to study the application of tianxia, equates Chinese dreams with dreams for the world.

Since the Olympics, Mr. Xi has used more direct tianxia language. “The Chinese have always held that the world is united and all under heaven are one family,” he declared in his 2017 New Year’s Message.

He made sure his revolutionary message was understood by having Foreign Minister Wang Yi explain it in Study Times, the influential Central Party School newspaper, in September 2017. “Xi Jinping thought on diplomacy,” Mr. Wang wrote, “has made innovations on and transcended the traditional Western theories of international relations for the past 300 years.” A “thought” in party lingo is an important body of ideology, such as “Mao Zedong Thought.”

Mr. Wang is almost certainly referring to the 1648 Treaty of Westphalia, which established the current international order by recognizing states as sovereign. When he says “transcended,” he hints that Mr. Xi aspires to a world without sovereign states—in other words, a unified world ruled by the Chinese.

As the Hudson Institute’s Charles Horner told me by email last week, many world leaders are nationalistic, but Mr. Xi is the only one whose “officially propounded nationalism takes the form of a global imperial vision.” That is consistent with his lawless behavior: treating neighbors as vassals, taking territory, closing off the global commons and intimidating leaders around the world.

“Tianxia,” Fei-Ling Wang notes, “inevitably and even necessarily makes the People’s Republic view and treat its neighbors and eventually all other states as essentially nonequals and lesser entities, to be influenced, controlled and subjugated with force, money, favor, ruse and fear.”

China is not, as some believe, a “trivial state” that seeks nothing more than to preserve its regime and defend its territory. With Mr. Xi pursuing tianxia ambitions, the world could use more of Mr. Trump’s defense of sovereignty, and even a little more “America First.” These concepts are not, as I once thought, unnecessarily provocative. They are a necessary defense of the centuries-old international order against an existential threat.

Mr. Chang is author of “The Coming Collapse of China.”