United States Secretary of Defense in the context of "There are unknown unknowns"

The United States Armed Forces are the military forces of the United States. U.S. federal law names six armed forces: the Army, Marine Corps, Navy, Air Force, Space Force, and Coast Guard. Since 1949, all of the armed forces, except the Coast Guard, have been permanently part of the United States Department of Defense, with the Space Force existing as a branch of the Air Force until 2019. They form six of the eight uniformed services of the United States.

From their inception during the American Revolutionary War, the Army and the Navy, and later the other services, have played a decisive role in the country's history. They helped forge a sense of national unity and identity through victories in the early-19th-century First and Second Barbary Wars. They played a critical role in the territorial evolution of the U.S., including the American Civil War. The National Security Act of 1947 created the Department of Defense or DoD, (after a short period being called the National Military Establishment) headed by the secretary of defense, superior to the service secretaries. It also created both the U.S. Air Force and National Security Council; in 1949, an amendment to the act merged the cabinet-level departments of the Army, Navy, and Air Force into the DoD.

The United States Army (U.S. Army) is the land service branch of the United States Armed Forces. It is designated as the Army of the United States in the United States Constitution. It operates under the authority, direction, and control of the United States secretary of defense. As a part of the United States Department of Defense, it is one of the six armed forces of the United States and one of the eight uniformed services of the United States. The Army is the most senior branch in order of precedence amongst the armed services. It has its roots in the Continental Army, formed on 14 June 1775 to fight against the British for independence during the American Revolutionary War (1775–1783). After the Revolutionary War, the Congress of the Confederation created the United States Army on 3 June 1784 to replace the disbanded Continental Army.

The U.S. Army is part of the Department of the Army, which is one of the three military departments of the Department of Defense. The U.S. Army is headed by a civilian senior appointed civil servant, the secretary of the Army (SECARMY), and by a chief military officer, the chief of staff of the Army (CSA) who is a member of the Joint Chiefs of Staff. It is the largest military branch, and in the fiscal year 2022, the projected end strength for the Regular Army (USA) was 480,893 soldiers; the Army National Guard (ARNG) had 336,129 soldiers and the U.S. Army Reserve (USAR) had 188,703 soldiers; the combined-component strength of the U.S. Army was 1,005,725 soldiers. The Army's mission is "to fight and win our Nation's wars, by providing prompt, sustained land dominance, across the full range of military operations and the spectrum of conflict, in support of combatant commanders". The branch participates in conflicts worldwide and is the major ground-based offensive and defensive force of the United States of America.‌

The United States Air Force (USAF) is the air service branch of the United States Armed Forces. It is a part of the United States Department of Defense and is one of the six armed forces and one of the eight uniformed services of the United States. Tracing its origins to 1 August 1907, as a part of the United States Army Signal Corps, the USAF was established by transfer of personnel from the Army Air Forces with the enactment of the National Security Act of 1947. It is the second youngest branch of the United States Armed Forces and the fourth in order of precedence. The United States Air Force articulates its core missions as air supremacy, global integrated intelligence, surveillance and reconnaissance, rapid global mobility, global strike, and command and control.

The Department of the Air Force, which serves as the USAF's headquarters and executive department, is one of the three military departments of the Department of Defense. The Department of the Air Force is headed by the civilian secretary of the Air Force, who reports to the secretary of defense and is appointed by the president with Senate confirmation. The highest-ranking military officer in the Air Force is the chief of staff of the Air Force, who exercises supervision over Air Force units and serves as one of the Joint Chiefs of Staff. As directed by the secretary of defense and secretary of the Air Force, certain Air Force components are assigned to unified combatant commands. Combatant commanders are delegated operational authority of the forces assigned to them, while the secretary of the Air Force and the chief of staff of the Air Force retain administrative authority over their members.

The United States Department of the Navy (DON) is one of the three military departments within the United States Department of Defense. It was established by an Act of Congress on 30 April 1798, at the urging of Secretary of War James McHenry, to provide a government organizational structure to the United States Navy (USN). Since 1834, the department has exercised jurisdiction over the United States Marine Corps (USMC), and during wartime the United States Coast Guard (USCG). These branches remain at all times independent and coequal service branches within the DON. It is led by the secretary of the Navy (SECNAV), a statutory civilian officer.

The Department of the Navy was an executive department, whose secretary served on the president's cabinet, until 1949, when amendments to the National Security Act of 1947 established the Department of Defense as a unified department for all military services; the DON, along with the Department of the Army and Department of the Air Force, became a component of the DoD, subject to the authority, direction and control of the secretary of defense.

The United States Department of the Air Force (DAF) is one of the three military departments within the Department of Defense of the United States of America. The Department of the Air Force was formed on September 18, 1947, per the National Security Act of 1947 (codified into Title 10 of the United States Code) and it is the military department within which the United States Air Force and the United States Space Force are organized.

The Department of the Air Force is headed by the secretary of the Air Force (SAF/OS), a civilian, who has the authority to conduct all of its affairs, subject to the authority, direction and control of the secretary of defense. The secretary of the Air Force's principal deputy is the under secretary of the Air Force (SAF/US). Their senior staff assistants in the Office of the Secretary of the Air Force are five assistant secretaries for acquisition, technology & logistics; financial management & comptroller; installations, environment & energy; manpower & reserve affairs; and space acquisition & integration and a general counsel. The highest-ranking military officers in the department, and senior military advisers to the secretary, are the chief of staff of the Air Force and chief of space operations.

The United States National Security Council (NSC) is the national security council used by the president of the United States for consideration of national security, military, and foreign policy matters. Based in the White House, it is part of the Executive Office of the President of the United States, and composed of senior national security advisors and Cabinet officials.

Since its inception in 1947 by President Harry S. Truman, the function of the council has been to advise and assist the president on national security and foreign policies. It also serves as the president's principal arm for coordinating these policies among various government agencies. The council has subsequently played a key role in most major events in U.S. foreign policy, from the Korean War to the war on terror.

The National Security Act of 1947 (Pub.L. 80-253, 61 Stat. 495, enacted July 26, 1947) was a law enacting major restructuring of the United States government's military and intelligence agencies following World War II. The majority of the provisions of the act took effect on September 18, 1947, the day after the Senate confirmed James Forrestal as the first secretary of defense.

The act merged the Department of the Army (renamed from the Department of War), the Department of the Navy, and the newly established Department of the Air Force (DAF) into the National Military Establishment (NME). The act also created the position of the secretary of defense as the head of the NME. It established the United States Air Force under the DAF, which worked to separate the Army Air Forces into its own service. It also protected the Marine Corps as an independent service under the Department of the Navy. Aside from the unification of the three military departments, the act established the National Security Council and the Central Intelligence Agency, the latter of which is headed by the director of central intelligence.

The chief of staff of the Army (CSA) is a statutory position in the United States Army held by a general officer. As the highest-ranking officer assigned to serve in the Department of the Army, the chief is the principal military advisor and a deputy to the secretary of the Army. In a separate capacity, the CSA is a member of the Joint Chiefs of Staff (10 U.S.C. § 151) and, thereby, a military advisor to the National Security Council, the secretary of defense, and the president of the United States. The CSA is typically the highest-ranking officer on active duty in the U.S. Army unless the chairman or the vice chairman of the Joint Chiefs of Staff are Army officers.

The chief of staff of the Army is an administrative position based in the Pentagon. While the CSA does not have operational command authority over Army forces proper (which is within the purview of the combatant commanders who report to the secretary of defense), the CSA does exercise supervision of army units and organizations as the designee of the secretary of the Army.