National security of the United States is a collective term encompassing the policies of both U.S. national defense and foreign relations. 
Measures taken to ensure U.S. national security include:
- Using diplomacy to rally allies and isolate threats.
- Marshaling economic power to elicit cooperation.
- Maintaining effective armed forces.
- Implementing civil defense and emergency preparedness policies (including anti-terrorism legislation)
- Ensuring the resilience and redundancy of critical infrastructure.
- Using intelligence services to detect and defeat or avoid threats and espionage, and to protect classified information.
- Tasking counterintelligence services or secret police to protect the nation from internal threats. 
The phrase “national security” entered U.S. political discourse as early as the Constitutional Convention. The Federalists argued that civilian control of the military required a strong central government under a single constitution. Alexander Hamilton wrote: “If a well-regulated militia be the most natural defense of a free country, it ought certainly to be under the regulation and at the disposal of that body which is constituted the guardian of the national security.” 
U.S. National Security organization has remained essentially stable since July 26, 1947, when U.S. President Harry S. Truman signed the National Security Act of 1947. Together with its 1949 amendment, this act:
- Created the National Military Establishment (NME) which became known as the Department of Defense when the act was amended in 1949.
- Formed a separate Department of the Air Force from the existing United States Army Air Forces.
- Subordinated the military branches to the new Secretary of Defense.
- Established the National Security Council to coordinate national security policy in the Executive Branch.
- Chartered the Central Intelligence Agency. 
After 9/11, the passage of the USA Patriot Act provoked debate about the alleged restriction of individual rights and freedoms for the sake of U.S. national security. The easing of warrant requirements for intelligence surveillance, under Title II of the Act, spurred the NSA warrantless surveillance controversy.  In August 2008, the United States Foreign Intelligence Surveillance Court of Review (FISCR) affirmed the constitutionality of warrantless national security surveillance. 
- National security#United States
- Anti-terrorism legislation
- Computer insecurity
- Homeland security
- Nuclear deterrence
- Terrorism in the United States
- U.S. Department of Defense, Joint Chiefs of Staff, New Revised Edition, Joint Pub. 1-02, 1990. Full text online
- Encyclopedia of United States National Security, 2 Vol., Sage Publications (2005), ISBN 0-7619-2927-4.
- Alexander Hamilton, The Federalist No. 29, “Concerning the Militia,” Jan. 9, 1788 Full text online
- Amy B. Zegart, Flawed by Design: The Evolution of the CIA, JCS, and NSC, Stanford University Press (1999, ISBN 0-8047-4131-X.
- Nola K Breglio, “Leaving FISA Behind: The Need to Return to Warrantless Surveillance,” Yale Law Journal, September 24, 2003. Full text PDF
- "Court Affirms Wiretapping Without Warrants," New York Times, January 15, 2009. Full text online.
- White House (20 May 2015). "The National Security Implications of a Changing Climate".