Team of Five, by Kate Brower: on Foreign Policy

Barack Obama: Obama to Trump: be kind, sustain international order

Obama's note warned Trump to "sustain the international order." He asked Trump to be kind, a request found in no other letters between presidents. "We've both been blessed, in different ways, with great good fortune," Obama wrote. "Not everyone is so lucky. It's up to us to do everything we can [to] build more ladders of success for every child and family that's willing to work hard." He ended the letter by wishing Trump and Melania, good luck and offering "to help in any ways which we can."

Jimmy Carter: Be flexible with North Korean leader on nuclear deal

Carter is the only former president Trump has sought advice from while in office. One quiet night in April 2019, a call came from a White House operator announcing that Trump wanted to talk to Carter about China and trade. Carter was able to ask about the status of talks with North Korea, and he advised the president to "be flexible" with the North Korean leader.

Later, when it was clear that Trump would not be sending him to broker any deal with the North Koreans--or anyone else--Carter didn't pull any punches. "I think he's a disaster," he said unequivocally. At a Carter Center event in June 2019, he went further, saying that a full investigation "would show that Trump didn't actually win the election in 2016. He was put into office because the Russians interfered."