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collective defense

a security arrangement whereby states or other entities agree to work together on defense, rather than each defending only itself. The principle of collective defense is enshrined in Article 5 of the North Atlantic Treaty, the founding document of the North Atlantic Treaty Organization (NATO), which states, “the Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all.” The parties also agree to help defend any allies that come under attack.