Simplify the following text for a Grade 8 native English-speaking person.
Original: Including his time serving as the 13th Secretary of Defense under Ford from 1975 to 1977 , Rumsfeld is the second-longest-serving Secretary of Defense in history , falling nine days short of the term of the longest-serving Pentagon chief , the Vietnam-era Secretary of Defense , Robert McNamara .
Simplified: Rumsfeld was the 13th United States Secretary of Defense under President Gerald Ford from 1975 to 1977 , and the 21st Secretary of Defense under President George W. Bush from 2001 to 2006 .