Collective Defense Clause
Concept Security and Defense Preferred
Treaty clause under which an armed attack on one party is treated as an attack on all parties.
Independent referenceNot an official or legal definition
Sources citedSee the sources at the end of this page
Status Under reviewLast updated 11 October 2026
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Definition
In plain language
A collective defence clause is a promise by allied countries that if one of them is attacked, the others will help. It is the core commitment of defensive alliances.
Academic definition
Article 5 of the North Atlantic Treaty (1949) is the best-known example: the parties agree that an armed attack against one or more of them shall be considered an attack against them all and that each will take such action as it deems necessary, including the use of armed force. The clause relies on the right of collective self-defence preserved by Article 51 of the UN Charter and does not oblige automatic military action, since each ally decides how to respond. Comparable clauses include Article 3 of the Rio Treaty (1947) and Article 42(7) of the Treaty on European Union, which was invoked by France in 2015.
Classification
- Term type
- Concept
- Category
- Security and Defense
- Subject field
- Security and Defense
- Subfield
- Alliance Commitments
- Difficulty level
- Intermediate
- Grammatical category
- noun phrase
Language variants
- Alternative terms
- mutual defence clause
- collective self-defence clause
- mutual assistance clause
- Non-preferred terms
- not_applicable
- Abbreviations
- not_applicable
- Acronyms
- not_applicable
- Native terms
- clause de défense collective (French)
- Transliteration
- not_applicable
- Pronunciation
- not_applicable
Context
Legal effect: Binding
Scope
Covers treaty commitments of mutual defence; broader collective security under the UN Charter, bilateral security guarantees and tripwire deployments are different.
Legal context
UN Charter Article 51 allows collective self-defence, and Article 51 requires measures to be reported to the Security Council. NATO Article 5 was invoked for the first time on 12 September 2001, and treaty clauses such as TEU Article 42(7) add a mutual assistance duty in the EU.
Diplomatic context
The clause supports deterrence, and debates over its credibility, scope and burden sharing recur in alliance summits and defence planning.
Institutional context
Operated by NATO's North Atlantic Council and military structure, the Organization of American States for the Rio Treaty, and the EU Council for Article 42(7).
Historical context
The Brussels Treaty of 1948 contained an early mutual defence clause, and NATO's Article 5 became the model after 1949.
Why it matters
The clause deters attack by making the cost of aggression higher but also creates the risk of drawing allies into conflicts.
Usage
Register: Legal
Usage note
British spelling 'defence' is used in some official texts, while the glossary title uses American 'defense'.
Example
The allies invoked the collective defense clause after the attack on one of their members.
Example scenario
State A, a member of an alliance, suffers an armed attack. The allies consult, agree that the collective defence clause applies and each contributes forces or support in the manner it chooses, with the action reported to the Security Council.
Common misuse
Saying that Article 5 requires automatic military response; it obliges each ally to take the action it deems necessary.
Not to be confused with
- Collective security system
- Security guarantee
- Mutual assistance in non-military matters
Sources
- The North Atlantic Treaty (1949), North Atlantic Treaty Organization, Article 5. nato.int
- Official definition. not_found_in_source
- Treaty reference. un.org
- Kaplan, NATO 1948: The Birth of the Transatlantic Alliance. not_found_in_source
- not_found_in_source. not_found_in_source
Source type: Treaty. Source reliability: Primary.
This entry is an independent reference definition, not an official or legally binding text. Where an official definition exists, it is linked above and is authoritative. Read our methodology.
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