One day in 1955, the fledgling People’s Republic of China was unhappy about U.S. involvement with Taiwan, the self-governing island that Beijing considered its territory. So Premier Zhou Enlai wrote a letter to the secretary-general of the United Nations.

“The Chinese people’s exercise of their own sovereign rights in liberating their own territory,” Zhou wrote, “is entirely a matter of China’s internal affairs.” He went on to use that wording, “internal affairs,” three more times in the letter.

Internal affairs. Across the decades, from Zhou’s relatively ineffectual plea extending to the dramatically different China that exists today, that precise phrase remains an indispensable tool for the Beijing government when challenges — and what it considers bad behavior — arrive from the outside.

Whether it’s foreign complaints about human rights, disputes over the South China Sea or anything about Taiwan or Xinjiang or Tibet and the Dalai Lama, the answer has been remarkably consistent: These are the internal affairs of China — so keep your nose out of them.

For evidence, just review this past week’s events and the Beijing government’s reactions.

A U.S. basketball team’s general manager supporting Hong Kong protesters with a single tweet from the United States? Interference in China’s internal affairs, with major repercussions for the NBA. Apple hosting an app to help those protesters locate police? Interference in internal affairs.

An episode of “South Park” that skewers Hollywood for cozying up to Beijing? Close enough to interference in internal affairs to get the show scrubbed from the Chinese internet.

And U.S. sanctions on Chinese companies that Washington says provide technology to repress minorities in the country’s predominantly Muslim Xinjiang region? No question about that one.

“I must point out that Xinjiang affairs are purely China’s internal affairs that allow no foreign interference,” Chinese Foreign Ministry spokesman Geng Shuang said Tuesday. “We urge the U.S. to correct its mistakes at once, withdraw this decision and stop its interference in China’s internal affairs.”

Ever since the Opium Wars in the mid-19th century, in which the British forced Qing Dynasty rulers into the opium trade and pried open Chinese ports to Western trade, China’s government has been particularly sensitive to Western doings inside its borders.

But tension between internal and external in China is far older. The very language it uses for itself and the rest of humanity illustrates that: “zhongguo,” or “middle country,” for China and “waiguo,” or “outside country,” for the foreign.

Like any government that controls lots of far-flung land, China, whether run by imperial rulers, Nationalists or the Communist Party, has almost always functioned as a central government trying mightily to maintain control at the edges of a sprawling territory.

In the U.S., power is balanced carefully between a federal government and the states. In China, the might always comes back to the nucleus of the nucleus — once the emperor inside the Forbidden City, now Xi Jinping and the Communist Party’s Politburo Standing Committee.

And in China’s case, watching the farthest reaches of its territory for incursions is a strand of the fundamental, historical DNA.