https://www.youtube.com/watch?v=8f2ROIPRWD0

Video Credit: PBS NewsHour

Thursday, January 12, marked day 3 of confirmation hearings for President-elect Donald Trump's cabinet picks. Among those that went before U.S. senators was retired Gen. James Mattis, Trump's pick to head the Department of Defense.

The secretary of Defense leads the largest government agency, with more than 1.3 million active duty military and nearly 700,000 civilian staff, in addition to 1.1 million citizens in the National Guard and Reserve forces.

The Department of Defense, commonly referred to as the “DOD,” provides the military forces to deter war and provide security for the nation, and is headquartered at the Pentagon in Arlington, VA. The DOD includes the Army, Navy, the Air Force, as well as Joint Chiefs of Staff, Pentagon Force Protection Agency, National Security Agency, and the Defense Intelligence Agency. The military and civilian arms of the DOD fight wars, provide humanitarian aid, perform peacekeeping, and provide relief services.