Question: Is the principal assistant to the president in all matters relating to the dod?

Disambiguated questions and answers:
context: The President exercises this supreme command authority through the civilian Secretary of Defense, who by federal law is the "head of the department", has "authority direction, and control over the Department of Defense", and is the "principal assistant to the President in all matters relating to the Department of Defense." The Secretary's principal deputy is the equally civilian Deputy Secretary of Defense who is delegated full powers to act for the Secretary of Defense. The Office of the Secretary of Defense (OSD) is the Secretary and Deputy Secretary's civilian staff, which includes several Under Secretaries and Assistant Secretaries of Defense with functional oversight responsibilities. The Secretaries of the Military Departments (i.e. Secretary of the Army, Secretary of the Navy, and Secretary of the Air Force) are subordinate to the Secretary of Defense. They have the authority under Title 10 of the United States Code to "conduct all the affairs" of their respective departments (Department of the Army, Department of the Navy, and Department of the Air Force) within which the military services are organized. Although subordinate to the Secretary of Defense, they are appointed by, and serve at the pleasure of, the President of the United States. As such, they cannot be removed from office by the Secretary of Defense, but can only be removed unilaterally by the President, or through impeachment by the Congress.
question: Is the principal assistant to the president in all matters relating to the dod in general?
short_answers: Secretary of Defense

The Secretary of Defense
wikipage: Organizational structure of the United States Department of Defense ...

context: James Norman Mattis (born September 8, 1950) is an American veteran and former government official who served as the 26th United States Secretary of Defense from January 2017 through December 2018. A retired United States Marine Corps general, Mattis served in the Persian Gulf War, the War in Afghanistan, and the Iraq War.
question: Is the principal assistant to the president in all matters relating to the dod from 2017 to 2018?
short_answers: James Norman Mattis

Jim Mattis
wikipage: Jim Mattis

context: No context provided
question: Is the principal assistant to the president in all matters relating to the dod from 2015 to 2017?
short_answers: Ash Carter

Dr. Ashton Baldwin Carter

context: Carter began his career as a physicist. After a brief experience as an analyst for the Congressional Office of Technology Assessment, he switched careers to public policy. He joined the Kennedy School of Government at Harvard University in 1984 and became chair of the International & Global Affairs faculty. Carter served as Assistant Secretary of Defense for International Security Policy during President Clinton's first term, from 1993 to 1996, responsible for policy regarding the former Soviet states, strategic affairs, and nuclear weapons. During President Obama's first term, he served first as Under Secretary of Defense for Acquisition, Technology and Logistics and then Deputy Secretary of Defense until December 2013. In February 2015, he replaced Chuck Hagel as Secretary of Defense and served until the end of the Obama administration.
question: Is the principal assistant to the president in all matters relating to the dod from 2013 to 2015?
short_answers: Chuck Hagel

Charles Timothy Hagel
wikipage: Ash Carter

Long answers:
long_answer: The Secretary of Defense, by federal law, is the "head of the department", has "authority direction, and control over the Department of Defense", and is the "principal assistant to the President in all matters relating to the Department of Defense." From 2013 to 2015, Chuck Hagel was the Secretary of Defense. Ash Carter held the role from 2015 to 2017, and Jim Mattis from 2017 to 2018.