Former Secretary of State Henry Kissinger, who has counseled numerous American presidents and statesmen since he left government in early 1977, is back in the halls of power once again. Since the election, he’s positioned himself close to Donald Trump, advising the president-elect on key appointments and praising him in public. And Kissinger, who has maintained close ties to Russian President Vladimir Putin, is now positioning himself as an intermediary between the Kremlin and the incoming Trump administration.

But previously undisclosed documents that I discovered while poring through the archives at Stanford’s Hoover Institution should give us serious pause about Kissinger’s resurgence. The storied former diplomat is not above using his considerable foreign policy credibility to further his private objectives, even to the detriment of the U.S. national interest. Indeed, on at least one occasion since he left public office, Kissinger used his influence with foreign leaders—in this case, the Pinochet regime in Chile—to undermine his domestic political opponents, including a sitting president of the United States.