The U.S. has maintained unofficial relations with Taiwan for nearly 40 years. At times, that's included the controversial sale of arms. CGTN's Roee Ruttenberg has more on Washington's policy, and how it's holding up under the Trump Administration.

In 1979, shortly after recognizing the People's Republic of China and establishing diplomatic ties with Beijing, Washington devised an OFFICIAL way to UNOFFICIALLY continue working directly with Taipei.

The U.S. Congress passed the Taiwan Relations Act, which declared it U.S. policy: "To preserve and promote extensive, close, and friendly commercial, cultural, and other relations between the people of the United States and the people on Taiwan."

So how did Washington go about doing just that? Douglas Paal lived in Taipei as the U.S.'s top representative.

DOUGLAS PAAL FORMER US ENVOY TO TAIWAN "They created this corporate entity called the American Institute in Taiwan, located in Virginia, and this company would receive money from Congress to do the business that embassies normally do, but this would not be called an embassy, the leader would not be called ambassador, and lots of fine distinctions were made to separate official from unofficial."

For decades, these distinctions seemed to satisfy Beijing's One-China policy. On occasions, the status quo was challenged.

Just weeks before being sworn-in as president, Donald Trump spoke by phone with Taiwan's leader Tsai Ing-wen. Though Trump was still a civilian at the time, the move prompted a quick rebuke from Beijing which saw it as a break in protocol. Months later, as president, Trump promised not to do it again, without first checking with China.

BONNIE GLASER CENTER FOR STRATEGIC AND INTERNATIONAL STUDIES "I think he's come to understand that is not the relationship we have with Taiwan."

Then, over the summer, the U.S. announced it was selling Taiwan nearly $1.5 billion worth of arms. Again, Beijing objected.

LU KANG CHINESE FOREIGN MINISTRY SPOKESPERSON "Taiwan is an inalienable part of China, and the United States selling weapons to Taiwan violates international law, the basic norms of international relations, and harms China's sovereignty and security interests."

BONNIE GLASER CENTER FOR STRATEGIC AND INTERNATIONAL STUDIES "When you have a president like Donald Trump, where there's an element of unpredictability, I think China wants to make it even clearer to the United States that it will not tolerate this kind of slippery slope."

The Taiwan Relations Act requires the U.S to: "Make available to Taiwan such defense articles and defense services to maintain a sufficient self-defense capacity." Just how much is "sufficient" is up to the U.S..

BONNIE GLASER CENTER FOR STRATEGIC AND INTERNATIONAL STUDIES "It also has an obligation to maintain its own very robust presence in the Western Pacific to prevent any coercion or intimidation or threats from Beijing."

But, she says: there's a limit.

BONNIE GLASER CENTER FOR STRATEGIC AND INTERNATIONAL STUDIES "There is no language in the Taiwan Relations Act that the United States is committed to come to Taiwan's defense if it is attacked. This is not a mutual defense treaty."

But some say that vagueness may be a good thing.

DOUGLAS PAAL FORMER US ENVOY TO TAIWAN "Sometimes the strategic ambiguity is a message to Beijing: don't raise the threat level militarily because you may find that our ambiguity actually resembles something closer to NATO defense arrangements. And we say to the Taiwanese: don't test us by provoking China to try to have us come in and defend you for something you shouldn't have done in the first place."

ROEE RUTTENBERG WASHINGTON DC For decades, the U.S. has refused to officially receive officials from Taiwan. But just days ago, the U.S. House of Representatives passed a bill - the Taiwan Travel Act - that would encourage exchanges at "all levels." The U.S. Senate still needs to consider it. But even the suggestion that it may happen - that the status quo may change -- has irked Beijing, which warned the U.S. against sending "any wrong signals". Roee Ruttenberg, CGTN, in Washington.