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Unlike Republican presidential candidates such as Trump, who is scheduled to meet with Kissinger today, and Mitt Romney in 2012, Clinton has done more than just pay him a visit before an election.
The two former secretaries of state have been close for years, with Clinton calling him "a friend" and noting that she "relied on his counsel" when she was secretary of state from 2009 to 2013.
"He checked in with me regularly, sharing astute observations about foreign leaders and sending me written reports on his travels. Though we have often seen the world and some of our challenges quite differently, and advocated different responses now and in the past...." she wrote in a review of his book "World Order" that was published in the Washington Post in 2014.
She also included mentions of Kissinger in her own book "Hard Choices," which detailed her time as secretary of state, referencing him during portions about China.
Clinton and Kissinger's ties extend beyond politics: The two are so friendly that when Clinton was set to present a fashion industry award to designer Oscar de la Renta, the event was re-arranged so that both Clinton and de la Renta were able to attend Kissinger's 90th birthday party in New York.
Though she has not spoken at length about her relationship with Kissinger during her ongoing presidential campaign, Clinton hasn't shied away from her connection to the controversial Republican either.
During a debate in New Hampshire in February, Clinton brought him up as a positive.
"I was very flattered when Henry Kissinger said I ran the State Department better than anybody had run in a long time," she said.
Sen. Bernie Sanders has criticized her ties to Kissinger, saying during a subsequent debate that he found it "rather amazing" that she is proud of the connection "because I happen to believe that Henry Kissinger was one of the most destructive secretaries of state in the modern history of this country."
"I'm proud to say that Henry Kissinger is not my friend. I will not take advice from Henry Kissinger," Sanders said before noting Kissinger's involvement in the bombing of Cambodia, which some critics have likened to genocide.