Donald Trump’s advisers spent weeks planning an unprecedented call between the President-elect and the president of Taiwan as part of a new strategy toward Taiwan, the Washington Post reported Sunday.

Per the Post:

The historic communication — the first between leaders of the United States and Taiwan since 1979 — was the product of months of quiet preparations and deliberations among Trump’s advisers about a new strategy for engagement with Taiwan that began even before he became the Republican presidential nominee, according to people involved in or briefed on the talks.

Stephen Yates, a former national security official for President George W. Bush who helped change language regarding China in the 2016 Republican Party platform, told the Washington Post that Taiwan’s president was on the list of officials for Trump to call “very early on.”

“Once the call was scheduled, I was told that there was a briefing for President-elect Trump. They knew that there would be reaction and potential blowback.” Yates said.

Trump’s call with the president of Taiwan marked the first call between leaders of the two countries since 1979. The call prompted China to issue a warning to Trump.

“It must be pointed out that there is only one China in the world,” the Chinese foreign ministry said in a Saturday statement directed at Trump. “We urge the relevant side in the U.S. to adhere to the ‘one China’ policy, abide by the pledges in the three joint China-U.S. communiques, and handle issues related to Taiwan carefully and properly to avoid causing unnecessary interference to the overall China-U.S. relationship.”

Trump defended the call on Twitter by claiming that the president of Taiwan called him. He then fired back at China on Sunday.

Did China ask us if it was OK to devalue their currency (making it hard for our companies to compete), heavily tax our products going into.. — Donald J. Trump (@realDonaldTrump) December 4, 2016