Kissinger left the State Department half a century ago. But he never left his old job.

When Henry Kissinger turned 100 this year, Secretary of State Antony J. Blinken toasted him at a birthday celebration in New York, and CIA Director William J. Burns did so at another birthday celebration in Washington. There was a reason for that: Kissinger managed to maintain his role as an adviser to Washington’s top policymakers half a century after his departure, often because what he did then was so relevant to today’s crises.

Mr. Kissinger spoke regularly with Mr. Blinken, including as recently as last month, Mr. Blinken said. He had also consulted with previous secretaries of state, including Condoleezza Rice, Hillary Clinton (who made a fuss about those conversations during her presidential campaign), John Kerry and Mike Pompeo. But he wasn’t a retired coach reminiscing about the good old days. Instead, he remained the ultimate backchanneler, especially for China’s leaders.

In July, Mr Kissinger secretly flew to China – by private jet, as it is an arduous flight even if you are not yet 100 years old – at the specific invitation of Xi Jinping, who called him an “old friend” mentioned and for a long time over dinner told him that “China-United States relations will forever be linked to the name ‘Kissinger.’”

It was a calculated move. Mr. Xi made clear that he wanted to return to the warmth that surrounded President Richard M. Nixon’s opening to China in the early 1970s, brokered by Mr. Kissinger in secret exchanges and a remarkable, also secret, trip to China. And the July visit helped set up Xi’s summit with President Biden outside San Francisco this month.

On that same trip, Mr. Kissinger was feted at the U.S. Embassy, ​​where R. Nicholas Burns, the current U.S. ambassador, lives in a house that Mr. Kissinger helped build when the United States had a representative in China, but entirely diplomatic. Recognition had not yet happened.

Mr. Kissinger met with the embassy’s extended staff and discussed what the process of opening the relationship looked like — in an era when it seemed unthinkable that China would become the world’s second-largest economy.

The Kissinger talks with secretaries of state and presidents were not just about navigating the downward spiral in relations with Beijing. He was involved in strategic discussions about Russia, with whom he negotiated SALT I, a major arms control treaty. He dabbled in artificial intelligence, a passion of his in recent years and a topic on which he wrote extensively, often together with Eric Schmidt, the former CEO of Google who became close to the former foreign minister.

To Mr. Kissinger’s many critics, this urge to stay involved, decades after he could have retired, reflected a thirst for power or an attempt to burnish a legacy, which he knew was tarnished by accusations that he forgave massacres, bombings and the deaths of thousands of people. when it served his diplomatic purposes.

But the reason he was asked for advice lies in the depth of his experience: When Mr. Kissinger died on Wednesday, Mr. Blinken was on his way to Israel in an effort to win an extended lull in a bloody conflict. Mr. Kissinger had walked the same path in November 1973, exactly 50 years ago, during his famous shuttle diplomacy.