NEW YORK • When Mrs Hillary Clinton was secretary of state, she relied on the counsel of her predecessor, Mr Henry Kissinger.

He would send her "astute observations about foreign leaders" and "written reports on his travels". She would joke with him that smartphones would have made his covert Cold War trip to Beijing impossible.

The two diplomats had a cordial, warm and respectful relationship, based on writings about their interactions during Mrs Clinton's tenure at the State Department.

"Kissinger is a friend, and I relied on his counsel when I served as secretary of state," Mrs Clinton wrote in The Washington Post, in a positive review of his book, World Order.

The friendship came back to haunt her in the Democratic presidential debate on Thursday night, when Senator Bernie Sanders pointedly questioned Mrs Clinton's foreign policy judgment, saying President Richard Nixon's secretary of state had enabled genocide in Cambodia under Pol Pot.

"I'm proud to say Henry Kissinger is not my friend," Mr Sanders said.

Mrs Clinton entered Thursday's debate under acute pressure to calm growing nervousness among her supporters after her drubbing by Mr Sanders in the New Hampshire primary and a razor-thin win in the Iowa caucus.

For his part, Mr Sanders hopes to harness the momentum and enthusiasm he gained from the first two contests and prove he can be a viable contender to lead the Democratic Party to victory in the Nov 8 presidential election.

To Mrs Clinton, Mr Kissinger, now 92, was part of "a fascinating club" of former secretaries of state she turned to when she arrived at Foggy Bottom, writing in her 2014 memoir, Hard Choices, that these diplomats "transcend partisan differences".

She praised Mr Kissinger's diplomatic achievements in China specifically, where he helped Mr Nixon normalise diplomatic relations with the Communist leadership in a visit to Beijing in 1972.

"I was riveted and proud of what America accomplished during what President Nixon called 'the week that changed the world'," she wrote.

Indeed, Mr Kissinger's trip looms over much of Mrs Clinton's experiences in China as President Barack Obama's top diplomat. She sought Mr Kissinger's advice when she met with the Chinese state councillor, Mr Dai Bingguo, and the Foreign Minister, Mr Yang Jiechi, who she said "would become my primary counterparts in the Chinese government".

"Henry Kissinger had told me how highly he valued his relationship with Dai, whom he found to be one of the most fascinating and open-minded Chinese officials he had ever encountered," Mrs Clinton wrote.

The squabbling over Mr Kissinger echoes the deep divisions over the Vietnam War during the early 1970s, when Mr Kissinger was a main target of the left for the Nixon administration's conduct of the war.

She also called on Mr Kissinger's advice in other areas, including asking for his help in Mr Obama's persuading of Prime Minister Benjamin Netanyahu of Israel to apologise in 2013 to President Recep Tayyip Erdogan of Turkey for the deaths of Turkish citizens in a Gaza flotilla three years earlier.

"I even enlisted Henry Kissinger to make the strategic case to him in August 2011," she wrote.

NEW YORK TIMES, REUTERS