Former secretary of state Henry Kissinger may be in his nineties, but he’s continuing to play a key, globe-spanning role in one of the most substantive foreign policy negotiations of the US presidency so far.

Kissinger, who brokered a ground-breaking detente between the US and China’s Communist Party’s in 1972, has served a valued go-between for the two nations for more than four decades, earning him the nickname of “old friend of the Chinese people.” It’s privilege he has shared with at least 600 people, although Kissinger may be the living person who has held the nickname the longest.

As recently as December, when then US president-elect Donald Trump threatened upheaval between the world’s most powerful nations, by accepting a congratulatory call from Taiwan’s Tsai Ing-wen, Kissinger was already in Beijing with Chinese president Xi Jinping, reassuring him that “overall, we hope to see the China-US relationship moving ahead in a sustained and stable manner.” (A Bloomberg report suggested that Xi may have turned to the venerable diplomat to better understand Trump, telling Kissinger he was “all ears” regarding what he had to say about the future of US-China relations.)

Kissinger met with the incoming Trump administration soon after the election, and helped to connect Chinese politicians with the US president’s son-in-law Jared Kushner, the Washington Post reports—connections that ultimately led to this week’s meeting.

In doing so, he’s opened up a now familiar controversy in the US—who does Kissinger work for, exactly, and whose side is he on?

Kissinger is “representing China’s interests and trying to influence American foreign policy,” said Craig Holman, a government affairs lobbyist for Public Citizen, a nonpartisan group that advocates for citizens’ rights in Congress. “That crosses the threshold for FARA,” he said, referring to the Foreign Agents Registration Act.

The act, passed in 1938, aimed to stop pro-Nazi agents from spreading propaganda inside the US, but was widened during the spread of Communism in Russia to include work for all foreign governments. Anyone who is acting for a foreign political party in the US, including public relations and lobbying work, must register as a foreign agent, something Kissinger has never done.

They are hardly new questions for Kissinger, who has served as the unofficial voice of the Chinese government in the West since he left the Gerald Ford administration in January 1977.

In 1997, Kissinger became a key advisor to a 1,000 strong corporate lobbyist group seeking better US-China relations, an arrangement that “strains the limits of lobbying disclosure laws and possibly violates the Foreign Agents Registration Act,” Justice Department officials said at the time.

“He has been doing this for a long time,” said Richard Painter, the former chief ethics officer for the George W. Bush administration said this week. The real question that needs to be answered is “What is the relationship between him and the foreign entity” he is advocating for, Painter said. “If he is doing this of his own accord, then no, I don’t think he’d have to register,” Painter said.

Kissinger & Associates, his firm in New York, did not respond to emailed questions.

Kissinger’s alliance with the Trump administration isn’t unexpected, since he’s made overtures to successive US administrations to act as a bridge-builder with China. Hillary Clinton relied on his counsel as secretary of state and said she considers him a “friend,” which drew outrage from the left. His presence has been a hard one to dismiss for many presidents, including former president George W. Bush, who controversially put him in charge of the 9/11 commission, a post he soon resigned from citing the difficult of resolving concerns over conflicts of interest.

Kissinger’s 600-page 2011 book “On China” is a bible for foreign businessmen and diplomats trying to work out political and trade deals in China, and it reads like the antithesis of the Trump administration’s approach to China so far, and a clue, perhaps, to Xi’s stance. As the New Republic review of the book summarizes: