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At the core of NATO's mutual defense commitment is Article 5 of the Washington Treaty, which states that an attack on one or more members of the alliance will be considered an attack on all. Also central are Article 2, which speaks of the members' commitment to their shared values and free institutions; Article 4, which provides for consultations if a member's security is threatened; and Article 10, which gives the members the option to invite additional states to join the alliance.
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NATO is based on the principle of “collective defense,” which means that an attack on one member is considered an attack on all members. The North Atlantic Treaty ... provides for formal consultations among all members if any one feels that its "territorial integrity, political independence or security" is threatened.
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