28 March 1967 – “U Thant was appointed as Secretary-General in 1961, six weeks after his predecessor Dag Hammarskjöld died in an air crash. In his first term, Thant facilitated negotiations between U.S. President John F. Kennedy and Soviet premier Nikita Khrushchev during the Cuban Missile Crisis of 1962, helping to avert a global catastrophe.
However, he fell out of favor when he “publicly criticized American conduct of the Vietnam War. His secret attempts at direct peace talks between Washington and Hanoi were eventually rejected by the Johnson Administration.”
Source: Wikipedia