Nicholas Katzenbach

Nicholas Katzenbach, who has died aged 90, was a lawyer and US government official who held influential posts in the Kennedy and Johnson administrations and played a dramatic, televised role in federal desegregation efforts in the South.

His eight years in government during the 1960s began with the idealism of Robert Kennedy's justice department and ended in the exhaustion and despair of a Lyndon B Johnson administration state department embroiled in the Vietnam war.

Nicholas deBelleville Katzenbach was born in Philadelphia to a family of politicians. He served in the US Army Air Force during the Second World War and spent two years as a prisoner of war in Italy and Germany, escaping twice but being recaptured on both occasions. He later graduated from Princeton University and Yale Law School and studied at Oxford for two years as a Rhodes Scholar.

For much of the 1950s he was a professor of law, first at Yale, then at the University of Chicago, and was one of the "brightest and best" brought into the Kennedy administration when John F Kennedy beat Richard Nixon in the 1960 presidential election. He was interviewed by attorney general Robert Kennedy (who addressed him as "Professor Katzenbach") and appointed to head the justice department's office of legal counsel.

Mr Katzenbach was the Kennedy administration's point man when, following a long court battle and subsequent violent protests, James Meredith became the first black person to enroll at the University of Mississippi in 1962. The following year he was the federal official on hand when segregationist Alabama Governor George Wallace tried to block two black students from entering the University of Alabama.

In front of the TV and press, Mr Katzenbach walked up to the school's entrance and handed Mr Wallace a presidential proclamation saying he must obey federal law. He later helped work on the 1964 Civil Rights Act signed into law by Lyndon Johnson.

Mr Katzenbach wrote a legal brief in support of JFK's decision to blockade Cuba during the 1962 missile crisis and helped secure the release of prisoners captured during the Bay of Pigs raid on Cuba in 1961. He became a deputy attorney general in 1963 and, after JFK's assassination, served as attorney general and an under-secretary of state under Mr Johnson, who was in Dallas with JFK when the president was shot.

Mr Katzenbach's first job for President Johnson was simple. On Air Force One returning from Dallas, Mr Johnson wanted to be sworn in as soon as possible. Mr Katzenbach, in Washington at the time, did not think such a ceremony was needed, but still agreed to speak to Johnson aide Jack Valenti and read to him the exact wording of the oath of office. The grim picture of Mr Johnson being sworn in, with Jacqueline Kennedy at his side, quickly became an iconic image of the transfer of power.

In the wake of the assassination, Mr Katzenbach was instrumental in the setting up of the Warren Commission, which concluded that gunman Lee Harvey Oswald acted alone, a theory still disputed.

In February 1965 Mr Johnson picked Mr Katzenbach as his Attorney General, but he held the post for less than two years after feuding with FBI director J Edgar Hoover.

A short time later, he was named an under-secretary of state, a post he held for the remainder of the Johnson administration and which led to an unhappy entanglement with the Vietnam War.

In 1969 Mr Katzenbach became IBM's general counsel. He also served on prison reform panels and remained active in national Democratic politics and constitutional issues. In December 1998 he took part in a protest in Princeton against Republican efforts to impeach President Bill Clinton and also spoke as a witness for the President. His 2008 memoir of his time in government was titled Some of It Was Fun.

He is survived by Lydia, his wife of 66 years, and their four children.