Who served as the Secretary of Defense?

James Mattis served as the Secretary of Defense from January 20, 2017, to January 1, 2019. In this role, he was responsible for overseeing the Department of Defense and was tasked with advising the President on military and defense matters. Mattis, a retired Marine Corps general, brought extensive military experience to the position, which is critical for making informed decisions regarding national security and military operations. His leadership during a time of global conflict and various international tensions emphasized the importance of military readiness and strategic partnerships.

The other individuals listed did not serve as Secretary of Defense. Rex W. Tillerson was the Secretary of State, overseeing diplomatic relations. Donald Trump was the President during Mattis's tenure and had a broader role in government leadership. General Joseph Dunford served as the Chairman of the Joint Chiefs of Staff, a key military role, but not as Secretary of Defense. Thus, the distinct role and responsibilities that James Mattis held confirm his position as the correct answer.