The United Earth Secretary of Defense was the member of the United Earth government responsible for coordinating the defense of the planet Earth and its colonies.
In 2155, during the Earth-Romulan War, Thomas Vanderbilt held this position and was responsible for advising the United Earth Prime Minister Nathan Samuels. (ENT novel: Beneath the Raptor's Wing)
By 2163, Althea Knowlton held the title after Vanderbilt was elected President of the Council of the United Federation of Planets. (ENT - Rise of the Federation novel: A Choice of Futures)