In this image provided by The Richard Nixon Library & Museum, shows two sides of a letter that Prince Philip wrote to President Richard Nixon. (The Richard Nixon Library & Museum via AP)

In this undated image provided by The Richard Nixon Library & Museum, Queen Elizabeth II and her husband Prince Philip talk with U.S. President Richard Nixon at Buckingham Palace in London. (The Richard Nixon Library & Museum via AP)

In this Nov. 4, 1969, image provided by The Richard Nixon Library & Museum, President Richard Nixon, center, stands with Prince Philip, second from left, prior to a black tie state dinner. (The Richard Nixon Library & Museum via AP)

In this undated image provided by The Richard Nixon Library & Museum, Queen Elizabeth II and her husband Prince Philip talk with U.S. President Richard Nixon at Buckingham Palace in London. (The Richard Nixon Library & Museum via AP)