collective defense

Collective defense is an arrangement, usually formalized by a treaty and an organization, among participant states that commit support in defense of a member state if it is attacked by another state outside the organization. NATO is the best known collective defense organization. Its now famous Article V calls on (but not fully commits) member states to assist another member under attack. This article was invoked after the September 11 attacks on the United States, after which other NATO members provided assistance to the US War on Terror in Afghanistan.
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