He was appointed as Secretary-General in 1961 , when 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 ( 1962 ) , helping to avert a global catastrophe . Later , in December 1962 , Thant ordered Operation Grandslam , which ended a secessionist insurgency in Congo . He was reappointed as Secretary-General on 2 December 1966 by a unanimous vote of the Security Council . In his second term , Thant was well known for publicly criticizing American conduct in the Vietnam War . He oversaw the entry of several newly independent African and Asian states into the UN . Thant refused to serve a third term and retired in 1971
died in an air crash
1961 Ndola United Nations DC-6 crash
U Thant