The **United States Secretary of Defense** is the highest-ranking official in the U.S. Department of Defense (DoD) and a key member of the President’s Cabinet, serving as the principal defense policy advisor to the President. Established by the National Security Act of 1947, this role was created to unify military command following the complexities of World Wars I and II. The Secretary oversees all branches of the U.S. military, managing a workforce of approximately 3.4 million military personnel and civilian employees, and holds significant authority over defense policies and military operations[1][2]. The Secretary of Defense is sixth in the presidential line of succession and works closely with a Deputy Secretary and the Secretaries of each military branch to formulate and implement defense strategies. The operational command chain runs from the President to the Secretary of Defense and then to the commanders of unified combatant commands, ensuring centralized control over military missions[1][3]. The Pentagon in Washington, D.C., serves as the department’s headquarters. Historically, notable Secretaries of Defense such as James Forrestal, the first appointee, Robert McNamara during the Vietnam War, and Donald Rumsfeld during the Iraq War have significantly shaped U.S. military strategy and policy[1]. The Secretary’s responsibilities include overseeing military readiness, budgeting, resource management, and coordinating with civilian and military leadership to address national security threats[2][5]. In 2025, the Department of Defense underwent a notable transition with the renaming of its head from Secretary of Defense to Secretary of War, with Pete Hegseth sworn in as the 29th Secretary. This change reflects evolving defense priorities but retains the core responsibility of managing the nation’s defense apparatus and advising top civilian decision-makers on policy and fiscal planning[4]. Overall, the Secretary of Defense plays a pivotal role at the intersection of national security, defense technology, and military operations, making it a critical organization for those interested in business, technology, and government affairs