Due to John Kerry's actions after his time in Vietnam he has forever disqualified himself from having any position of authority over the men and women of the armed forces of the United States of America.
Testifying before the Senate Foreign Relations Committee on April 23, 1971 John Kerry made it clear that he held disdain for the military and expressly commented that members of the armed forces are not the best the country has to offer. Later the same day he would lead a protest where veterans threw their medals over a fence showing they had no interest in the military nor their status as veterans.
For these, and myriad other reasons, John F. Kerry is not fit to serve as the Secretary of Defense.