James Mattis

This retired Marine, whose nicknames include "Warrior Monk" and "Mad Dog," is known for having a hard-line approach to America's enemies, particularly Iran. Mattis led operations across the Middle East less than four years ago and served for more than 40 years in the military. The general is one of the most influential military leaders of the age. Since retiring he has  served as a strategic thinker, consultant, and fellow for a think tank at Stanford University called the Hoover Institution.  
In order for him to take the job, Congress will have to bypass a federal law stating that defense secretaries cannot have been on active duty within the previous seven years. The last time congress made an exception was more than fifty years ago when Gen. George C. Marshall became defense secretary in 1950.