The US's One-China Policy Explained A look at the long-standing China policy Trump has now acknowledged.

 -- Then-President-elect Donald Trump's unprecedented phone conversation with the leader of Taiwan in December, followed by his anti-China tweets, signaled strongly that upon becoming president he could seek to deviate from America's long-standing one-China policy.

Now in office, however, Trump has reversed course, confirming his commitment to honor the United States’ long-standing one-China policy, according to the White House's characterization of a phone conversation Thursday night with Chinese president Xi Jinping.

"The two leaders discussed numerous topics and President Trump agreed, at the request of President Xi, to honor our 'one China" policy," the White House said.

Trump's December phone conversation with Taiwanese President Tsai Ing-wen prompted headlines across the United States that Trump had broken with a decades-old policy and even forced the Obama administration to respond and affirm its commitment to the one-China current policy.

In the months since that call, the China-U.S. relationship has been strained. So what is the U.S. position on China and Taiwan, and why is it so delicate?

One-China, Briefly

Since the signing of the Taiwan Relations Act in 1979, it has been the policy of the United States to recognize Taiwan as part of China.

"The United States does not support Taiwan independence," reads a State Department fact sheet updated just this September. But, it continues, "maintaining strong, unofficial relations with Taiwan is a major U.S. goal, in line with the U.S. desire to further peace and stability in Asia."

Taiwan is the United States' ninth-largest trading partner, and according to State Department figures, Taiwan employed more than 12,000 workers in the United States and paid them nearly $1 billion. The one-China policy amounts to a delicate balance between respecting China's claim to the territory and maintaining close ties to Taiwan.

Missiles Pointed

The U.S. commitment to Taiwan permits the sale of defensive weapons, and last year the U.S. sold Taipei $1.83 billion worth, most of which it uses to defend the island from a potential provocations from Beijing.

That last package, the first of its kind in four years, consisted of two Oliver Hazard Perry class Navy frigates, Javelin anti-tank missiles, amphibious assault vehicles, anti-aircraft missiles, anti-ship systems and other equipment.

Since 1979, the U.S. has made $12 billion in weapons sales to Taiwan, and Beijing voices its opposition every time.

A State Department official told ABC News in December that the United States is constantly reviewing Taiwan's defensive needs and requests and will continue to do so. "We do not consult Beijing about our military cooperation with or arms sales to Taiwan," this official said.

Despite the weapons sales and the importance of good relations emphasized by both sides, Taiwan is not a treaty ally with the United States, and the U.S. has no obligation to defend it if it is attacked.

Trump's Approach

ABC News learned in December that Trump's congratulatory call from Tsai was expected and likely arranged by people in his transition team. So while it was a surprise to China and many in the U.S. government, Trump's advisers claimed it was calculated.

Trump took a firm stance on confronting a rising China during his campaign, repeatedly accusing China of manipulating its currency and once saying, "We already have a trade war [with China]," suggesting he doesn't fear an escalation.

He also floated the idea of imposing major tariffs on Chinese products. "We have the power over China, economic power, and people don't understand it," he said in April.

After taking office, Trump issued an executive order withdrawing the United States from the Trans Pacific Partnership (TPP), which effectively leaves the future trade relationship with China unclear.

Chinese state media in recent weeks had increasingly called for the United States to recognize the one-China policy. Trump's acknowledgement of it will undoubtedly calm Chinese fears that a Trump presidency could mean a major upheaval in U.S.-China relations.