Jim Mattis became the 26th defense secretary Jan. 20, 2017. During more than four decades in uniform, he commanded Marines at all levels, including in combat tours in Iraq and Afghanistan. Following his retirement from the U.S. Marine Corps in 2013, Secretary Mattis served as the Davies Family Distinguished Visiting Fellow at the Hoover Institution at Stanford University, specializing in the study of leadership, national security, strategy, innovation and the effective use of military force.