Why must every NATO member approve a new country’s accession protocols?

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Flag of NATO featuring a white compass rose emblem with four lines radiating from the center on a blue background. — North Atlantic Treaty Organization Flag of the North Atlantic Treaty Organization (NATO).

Every North Atlantic Treaty Organization (NATO) member must approve a new country’s accession protocols because the alliance is built on a collective defense commitment. Under Article 5 of the North Atlantic Treaty, all member states are obligated to defend any other member in the event of an armed attack.12

Because admitting a new nation expands this significant security guarantee to include the new member’s territory, the existing members must unanimously agree to the inclusion. The ratification of these accession protocols is a political decision, and because it carries such profound military and strategic consequences, any individual member-state may choose to withhold its approval for any reason, including unrelated political grievances.1