Originally conceived as protection against military attack, national security is now widely understood to include non-military dimensions, including the security from terrorism, crime, economic security, energy security, environmental security, food security, cyber security etc. Similarly, national security risks include, in addition to the actions of other nation states, action by violent non-state actors, narcotic cartels, and multinational corporations, and also the effects of natural disasters.
Governments rely on a range of measures, including political, economic, and military power, as well as diplomacy to safeguard the security of the nation state. They may also act to build the conditions of security regionally and internationally by reducing transnational causes of insecurity, such as climate change, economic inequality, political exclusion, and nuclear proliferation.
The concept of national security remains ambiguous, having evolved from simpler definitions which emphasised freedom from military threat and from political coercion.    Among the many definitions proposed to date are the following, which show how the concept has evolved to encompass non-military concerns:
- "A nation has security when it does not have to sacrifice its legitimate ínterests to avoid war, and is able, if challenged, to maintain them by war." (Walter Lippmann, 1943). 
- "The distinctive meaning of national security means freedom from foreign dictation." (Harold Lasswell, 1950) 
- "National security objectively means the absence of threats to acquired values and subjectively, the absence of fear that such values will be attacked." (Arnold Wolfers, 1960)
- "National security then is the ability to preserve the nation's physical integrity and territory; to maintain its economic relations with the rest of the world on reasonable terms; to preserve its nature, institution, and governance from disruption from outside; and to control its borders." (Harold Brown, U.S. Secretary of Defense, 1977-1981)
- "National security... is best described as a capacity to control those domestic and foreign conditions that the public opinion of a given community believes necessary to enjoy its own self-determination or autonomy, prosperity and wellbeing." (Charles Maier, 1990)
- "National security is an appropriate and aggressive blend of political resilience and maturity, human resources, economic structure and capacity, technological competence, industrial base and availability of natural resources and finally the military might." (National Defence College of India, 1996)
- "[National security is the] measurable state of the capability of a nation to overcome the multi-dimensional threats to the apparent well-being of its people and its survival as a nation-state at any given time, by balancing all instruments of state policy through governance... and is extendable to global security by variables external to it." (Prabhakaran Paleri, 2008) 
- "[National and international security] may be understood as a shared freedom from fear and want, and the freedom to live in dignity. It implies social and ecological health rather than the absence of risk... [and is] a common right." (Ammerdown Group, 2016) 
Dimensions of national security
Potential causes of national insecurity include actions by other states (e.g. military or cyber attack), violent non-state actors (e.g. terrorist attack), organised criminal groups such as narcotic cartels, and also the effects of natural disasters (e.g. flooding, earthquakes).   Systemic drivers of insecurity, which may be transnational, include climate change, economic inequality and marginalisation, political exclusion, and militarisation.
In view of the wide range of risks, the security of a nation state has several dimensions, including economic security, energy security, physical security, environmental security, food security, border security, and cyber security. These dimensions correlate closely with elements of national power.
Increasingly, governments organise their security policies into a national security strategy (NSS); as of 2017, Spain, Sweden, the United Kingdom, and the United States are among the states to have done so. Some states also appoint a National Security Council to oversee the strategy and/or a National Security Advisor.
Although states differ in their approach, with some beginning to prioritise non-military action to tackle systemic drivers of insecurity, various forms of coercive power predominate, particularly military capabilities. The scope of these capabilities has developed. Traditionally, military capabilities were mainly land- or sea-based, and in smaller countries they still are. Elsewhere, the domains of potential warfare now include the air, space, cyberspace, and psychological operations. Military capabilities designed for these domains may be used for national security, or equally for offensive purposes, for example to conquer and annex territory and resources.
In practice, national security is associated primarily with managing physical threats and with the military capabilities used for doing so. That is, national security is often understood as the capacity of a nation to mobilise military forces to guarantee its borders and to deter or successfully defend against physical threats including military aggression and attacks by non-state actors, such as terrorism. Most states, such as South Africa and Sweden, configure their military forces mainly for territorial defence; others, such as France, Russia, the UK and the US, invest in higher-cost expeditionary capabilities, which allow their armed forces to project power and sustain military operations abroad.
Barry Buzan, Ole Wæver, Jaap de Wilde and others have argued that national security depends on political security: the stability of the social order. Others, such as Paul Rogers, have added that the equitability of the international order is equally vital. Hence, political security depends on the rule of international law (including the laws of war), the effectiveness of international political institutions, as well as diplomacy and negotiation between nations and other security actors. It also depends on, among other factors, effective political inclusion of disaffected groups and the human security of the citizenry.
Economic security, in the context of international relations, is the ability of a nation state to maintain and develop the national economy, without which other dimensions of national security cannot be managed. In larger countries, strategies for economic security expect to access resources and markets in other countries, and to protect their own markets at home. Developing countries may be less secure than economically advanced states due to high rates of unemployment and underpaid work.
Ecological security, also known as environmental security, refers to the integrity of ecosystems and the biosphere, particularly in relation to their capacity to sustain a diversity of life-forms (including human life). The security of ecosystems has attracted greater attention as the impact of ecological damage by humans has grown. The degradation of ecosystems, including topsoil erosion, deforestation, biodiversity loss, and climate change, affect economic security and can precipitate mass migration, leading to increased pressure on resources elsewhere.
The scope and nature of environmental threats to national security and strategies to engage them are a subject of debate.  Romm (1993) classifies the major impacts of ecological changes on national security as: 
- Transnational environmental problems. These include global environmental problems such as climate change due to global warming, deforestation, and loss of biodiversity. 
- Local environmental or resource pressures. These include resource scarcities leading to local conflict, such as disputes over water scarcity in the Middle East; migration into the United States caused by the failure of agriculture in Mexico;  and the impact on the conflict in Syria of erosion of productive land. Environmental insecurity in Rwanda following a rise in population and dwindling availability of farmland, may also have contributed to the genocide there.
- Environmentally threatening outcomes of warfare. These include acts of war that degrade or destroy ecosystems. Examples are the Roman destruction of agriculture in Carthage; Saddam Hussein's burning of oil wells in the Gulf War;  the use of Agent Orange by the UK in the Malayan Emergency and the USA in the Vietnam War for defoliating forests; and the high greenhouse gas emissions of military forces.
Resources include water, sources of energy, land and minerals. Availability of adequate natural resources is important for a nation to develop its industry and economic power. For example, in the Persian Gulf War of 1991, Iraq captured Kuwait partly in order to secure access to its oil wells, and one reason for the US counter-invasion was the value of the same wells to its own economy. Water resources are subject to disputes between many nations, including India and Pakistan, and in the Middle East.
The interrelations between security, energy, natural resources, and their sustainability is increasingly acknowledged in national security strategies and resource security is now included among the UN Sustainable Development Goals. In the US, for example, the military has installed solar photovoltaic microgrids on their bases in case of power outage.
Computer security, also known as cybersecurity or IT security, refers to the security of computing devices such as computers and smartphones, as well as computer networks such as private and public networks, and the Internet. It concerns the protection of hardware, software, data, people, and also the procedures by which systems are accessed, and the field has growing importance due to the increasing reliance on computer systems in most societies. Since unauthorized access to critical civil and military infrastructure is now considered a major threat, cyberspace is now recognised as a domain of warfare.
Infrastructure security is the security provided to protect infrastructure, especially critical infrastructure, such as airports, highways  rail transport, hospitals, bridges, transport hubs, network communications, media, the electricity grid, dams, power plants, seaports, oil refineries, and water systems. Infrastructure security seeks to limit vulnerability of these structures and systems to sabotage, terrorism, and contamination.
Many countries have established government agencies to directly manage the security of critical infrastructure usually through the Ministry of Interior/Home Affairs, dedicated security agencies to protect facilities such as United States Federal Protective Service, and also dedicated transport police such as the British Transport Police. There are also commercial transportation security units such as the Amtrak Police in the United States. Critical infrastructure is vital for the essential functioning of a country. Incidental or deliberate damage can have a serious impact on the economy and essential services. Some of the threats to infrastructure include:
- Terrorism: person or groups deliberately targeting critical infrastructure for political gain. In the November 2008 Mumbai attacks, the Mumbai central station and hospital were deliberately targeted, for example.
- Sabotage: person or groups such as ex-employees, anti-government groups, environmental groups. Refer to Bangkok's International Airport Seized by Protestors.
- Information warfare: private person hacking for private gain or countries initiating attacks to glean information and damage a country's cyber infrastructure. Cyberattacks on Estonia and cyberattacks during the 2008 South Ossetia war are examples.
- Natural disaster: hurricane or other natural events which damage critical infrastructure such as oil pipelines, water and power grids. See Hurricane Ike and Economic effects of Hurricane Katrina for examples.
Issues in national security
The dimensions of national security outlined above are frequently in tension with one another. For example:
- The high cost of maintaining large military forces places a burden on the economic security of a nation. The share of government expenditure on state armed forces varies internationally; for example, in 2015 it was 4% in Germany, 9% in Chile, 14% in the USA, 15% in Israel, and 19% in Pakistan. Conversely, economic constraints can limit the scale of expenditure on military capabilities.
- Unilateral security action by states can undermine political security at an international level if it erodes the rule of law and undermines the authority of international institutions. The invasion of Iraq in 2003 and the annexation of Crimea in 2014 have been cited as examples.
- The pursuit of economic security in competition with other nation states can undermine the ecological security of all when the impact includes widespread topsoil erosion, biodiversity loss, and climate change. Conversely, expenditure on mitigating or adapting to ecological change places a burden on the national economy.
If tensions such as these are not managed effectively, national security policies and actions may be ineffective or counterproductive.
Increasingly, national security strategies have begun to recognise that nations cannot provide for their own security without also developing the security of their regional and international context. For example, Sweden's national security strategy of 2017 declared:
The extent to which this matters, and how it should be done, is the subject of debate. Some argue that the principal beneficiary of national security policy should be the nation state itself, which should centre its strategy on protective and coercive capabilities in order to safeguard itself in a hostile environment (and potentially to project that power into its environment, and dominate it to the point of strategic supremacy). Others argue that security depends principally on building the conditions in which equitable relationships between nations can develop, partly by reducing antagonism between actors, ensuring that fundamental needs can be met, and also that differences of interest can be negotiated effectively. In the UK, for example, Malcolm Chalmers argued in 2015 that the heart of the UK's approach should be support for the Western strategic military alliance led through NATO by the United States, as "the key anchor around which international order is maintained". The Ammerdown Group argued in 2016 that the UK should shift its primary focus to building international cooperation to tackle the systemic drivers of insecurity, including climate change, economic inequality, militarisation and the political exclusion of the world's poorest people.
Approaches to national security can have a complex impact on human rights and civil liberties. For example, the rights and liberties of citizens are affected by the use of military personnel and militarised police forces to control public behaviour; the use of surveillance, including mass surveillance in cyberspace, which has implications for privacy; military recruitment and conscription practices; and the effects of warfare on civilians and civil infrastructure. This has led to a dialectical struggle, particularly in liberal democracies, between government authority and the rights and freedoms of the general public.
Even where the exercise of national security is subject to good governance and the rule of law, a risk remains that the term national security may be become a pretext for suppressing unfavorable political and social views. In the US, for example, the controversial USA Patriot Act of 2001, and the revelation by Edward Snowden in 2013 that the National Security Agency harvests the personal data of the general public, brought these issues to wide public attention. Among the questions raised are whether and how national security considerations at times of war should lead to the suppression of individual rights and freedoms, and whether such restrictions are necessary when a state is not at war.
National Security ideology as taught by the US Army School of the Americas to military personnel were vital in causing the military coup of 1964. The military dictatorship was installed on the claim by military that Leftists were an existential threat to the national interests.
The concept of national security became an official guiding principle of foreign policy in the United States when the National Security Act of 1947 was signed on July 26, 1947 by U.S. President Harry S. Truman.  As amended in 1949, this Act:
Notably, the Act did not define national security, which was conceivably advantageous, as its ambiguity made it a powerful phrase to invoke whenever issues threatened by other interests of the state, such as domestic concerns, came up for discussion and decision. 
The notion that national security encompasses more than just military security was present, though understated, from the beginning. The Act established the National Security Council so as to "advise the President on the integration of domestic, military and foreign policies relating to national security". 
While not defining the "interests" of national security, the Act does establish, within the National Security Council, the "Committee on Foreign Intelligence", whose duty is to conduct an annual review "identifying the intelligence required to address the national security interests of the United States as specified by the President" (emphasis added).
To reflect on institutionalization of new bureaucratic infrastructures and governmental practices in the post-World War II period in the U.S., when a culture of semi-permanent military mobilization brought around the National Security Council, the CIA, the Department of Defense, and the Joint Chiefs of Staff, national-security researchers apply a notion of a national security state:
In 2010, the White House included an all-encompassing world-view in a national security strategy which identified "security" as one of the country's "four enduring national interests" that were "inexorably intertwined":
U.S. Secretary of State Hillary Clinton has said that, "The countries that threaten regional and global peace are the very places where women and girls are deprived of dignity and opportunity". She has noted that countries where women are oppressed are places where the "rule of law and democracy are struggling to take root", and that, when women's rights as equals in society are upheld, the society as a whole changes and improves, which in turn enhances stability in that society, which in turn contributes to global society.
In the United States, the Bush Administration in January 2008, initiated the Comprehensive National Cybersecurity Initiative (CNCI). It introduced a differentiated approach, such as: identifying existing and emerging cybersecurity threats, finding and plugging existing cyber vulnerabilities, and apprehending actors that trying to gain access to secure federal information systems. President Obama issued a declaration that the "cyber threat is one of the most serious economic and national security challenges we face as a nation" and that "America's economic prosperity in the 21st century will depend on cybersecurity."
China's Armed Forces are known as the People's Liberation Army (PLA). The military is largest in the world with 2.3 million active troops in 2005.
The Ministry of State Security was established in 1983 to ensure “the security of the state through effective measures against enemy agents, spies, and counterrevolutionary activities designed to sabotage or overthrow China’s socialist system.”
Muslim separatists in Xinjiang Uygur Autonomous Region are China's most significant domestic threat.
State of the national security of Republic of India is determined by its internal stability and geopolitical interests. While Islamic upsurge in Indian State of Jammu and Kashmir demanding secession and far left wing terrorism in India's red corridor remain some key issues in India's internal security, terrorism from Pakistan based militant groups has been emerging as a major concern for New Delhi.
National Security Advisor of India heads National Security Council of India, receives all kinds of intelligence reports and is chief advisor to the Prime Minister of India over national and international security policy. National Security Council has India's defence, foreign, home, finance ministers and deputy chairman of NITI Aayog as its members and is responsible for shaping strategies for India's security in all aspects.
In the years 1997 and 2000, Russia adopted documents titled "National Security Concept" that described Russia's global position, the country's interests, listed threats to national security and described the means to counter those threats. In 2009, these documents were superseded by the "National Security Strategy to 2020". The key body responsible for coordination of policies related to Russia's national security is the Security Council of Russia.
According to provision 6 of the National Security Strategy to 2020, national security is "the situation in which the individual, the society and the state enjoy protection from foreign and domestic threats to the degree that ensures constitutional rights and freedoms, decent quality of life for citizens, as well as sovereignty, territorial integrity and stable development of the Russian Federation, the defense and security of the state."
The primary body responsible for coordinating national security policy in the UK is the National Security Council (United Kingdom) which helps produce and enact the UK's National Security Strategy. It was created in May 2010 by the new coalition government of the Conservative Party (UK) and Liberal Democrats. The National Security Council is a committee of the Cabinet of the United Kingdom and was created as part of a wider reform of the national security apparatus. This reform also included the creation of a National Security Adviser and a National Security Secretariat to support the National Security Council.
Conceptualizing and understanding the National Security choices and challenges of African States is a difficult task. This is due to the fact that it is often not rooted in the understanding of their (mostly disrupted) state formation and their often imported process of state building.
Although Post-Cold War conceptualizations of Security have broadened, the policies and practices of many African states still privilege national security as being synonymous with state security and even more narrowly- regime security.
The problem with the above is that a number of African states (be specific) have been unable to govern their security in meaningful ways. Often failing to be able to claim the monopoly of force in their territories. The hybridity of security ‘governance’ or ‘providers’ thus exists. States that have not been able to capture this reality in official National Security strategies and policies often find their claim over having the monopoly of force and thus being the Sovereign challenged. This often leads to the weakening of the state. Examples of such states are South Sudan and Somalia.