Hong Kong (CNN) "We do not want to get in the middle of the US politics."

That was the reaction of a Chinese diplomat to the revelation that United States President Donald Trump had urged Beijing to investigate Democratic presidential candidate Joe Biden and his son Hunter for their business dealings in China

But the diplomat wasn't simply expressing a desire to avoid what is turning into an increasingly messy scandal that could dominate the run-up to the 2020 election -- but a foundational tenet of Chinese foreign policy.

In a call with Chinese President Xi Jinping, Trump seemed to confirm China's worst suspicions about the US in this regard. According to two people familiar with the discussion , Trump promised to remain quiet on ongoing anti-government unrest in Hong Kong if trade talks progressed ( they haven't, and he hasn't ).

Whether China actually sticks to its principles of non-interference is highly debatable. But the idea that Beijing would ever agree openly to a tit-for-tat deal on something like the Biden investigation represents a major misunderstanding of how Chinese politics works, and perhaps provides a clue why Trump has failed to make any breakthroughs in other areas with Beijing.

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Self-serving policy

China's policy of non-interference dates back to the Sino-Soviet split of 1956, which redefined the Cold War as a tripolar dispute and set the stage for rapprochement between Beijing and Washington.

Under the Five Principles of Peaceful Coexistence , China -- along with other non-aligned powers like India and Myanmar -- agreed to uphold "mutual respect for sovereignty and territorial integrity, mutual non-aggression, non-interference in each other's internal affairs, equality and mutual benefit, and peaceful coexistence."

But despite many of the other principles being openly contravened, non-interference has remained a key public tenet of Chinese foreign policy -- both in terms of what it claims to practice, and what it demands of allies and rivals.

At the United Nations for example, where the People's Republic of China has sat on the Security Council since 1971, Beijing has worked to "move away from human rights-related civil developmental goals to economic development as the main aim," according to a recent report on China's role at the UN.

It has also repeatedly voted against any external interference in other countries' affairs. Beijing was a strong critic of the US invasion of Iraq in 2003, though it initially supported action against Saddam Hussein in the 1990s following Iraq's invasion of Kuwait.

In 2007, Beijing joined with Moscow in vetoing any criticism of Myanmar on its human rights record; the following year both also vetoed sanctions against Zimbabwe , as they have multiple times with regard to Syria . China has also been the strongest check at the UN on any action against North Korea's nuclear ambitions, restraining what the international community can do and when.

This has remained consistent even as Beijing has increased its own role in peacekeeping efforts overseas, expanded its military footprint to Africa and throughout Asia, and lobbied country after country to drop support for Taiwan in favor of China

"China appears to have struck a most convenient -- and shrewd -- balance of selective legitimate foreign intervention and soft power efforts, while at the same time maintaining the capacity to both veto Security Council resolutions as it sees fit and shake off any reproach of its domestic policies and strategies," according to Sherif A. Elgebeily, director of the London-based Center for the Study of International Peace and Security.

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Tactical misstep

While it is often couched in noble terms, the principle of non-interference is all about China avoiding criticism of or meddling in its own policies. From Beijing's perspective, whatever other countries think about its actions in Xinjiang, Tibet or Hong Kong, their responsibility or ability to do anything about them stops at China's borders.

Speaking at the United Nations last month, Foreign Minister Wang Yi spelled this out , saying "if the China-US relationship is to remain stable, it is most important that we respect each other's territorial sovereignty, social system and development path, and try not to impose one's will or model on the other."

"China will never interfere in the internal affairs of the United States, and we trust that the American people are capable of sorting out their own problems," Wang said. "Likewise, we expect the US to treat China in the same spirit and not interfere in China's internal affairs."

That's why it is so bizarre -- and such a potential tactical misstep -- for Trump to ask China, both in private and now openly, to investigate one of his potential electoral rivals.

Beijing may not stick to its principles in practice, and indeed there is plenty of evidence that China runs the type of influence campaigns and behind-the-scenes diplomacy that both Moscow and Washington are adept at, but it is never going to acknowledge this publicly or agree to a tit-for-tat deal.

Doing so would open the door to foreign interference in China's own affairs, one that Beijing has spent decades trying to jam shut as hard as possible.