In 1960 , Cooper was re-elected , securing his first full , six-year term in the Senate . Newly elected President John F. Kennedy – Cooper 's former Senate colleague – chose Cooper to conduct a secret fact-finding mission to Moscow and New Delhi . Following Kennedy 's assassination in November 1963 , President Lyndon B. Johnson appointed Cooper to the Warren Commission to investigate the assassination . Cooper soon became an outspoken opponent of Johnson 's decision to escalate U.S. military involvement in the Vietnam War , consistently advocating negotiation with the North Vietnamese instead . After Cooper 's re-election in 1966 , he worked with Idaho Democrat Frank Church on a series of amendments designed to de-fund further U.S. military operations in the region . These amendments were hailed as the first serious attempt by Congress to curb presidential authority over military operations during an ongoing war . Aging and increasingly deaf , Cooper did not seek re-election in 1972 . His last acts of public service were as Ambassador to East Germany from 1974 to 1976 and as an alternate delegate to the United Nations in 1981 . He died in a Washington , D.C. , retirement home on February 21 , 1991 , and was buried in Arlington National Cemetery
Kennedy 's assassination
Assassination of John F. Kennedy
John Sherman Cooper