Quite simply, nations have to meet certain criteria to join. NATO isn't just a protection club. Nations are expected to contribute their fair share to the collective military strength, intelligence, and finances. Member nations are trusted with NATO technology and NATO secrets. While anyone is free to apply for membership, actually being accepted isn't so easy.
Besides, why would we allow countries in South America or Africa to join? This particular alliance is about thwarting Russian (previously Soviet) aggression. We can have other alliances with other purposes. For example, Japan isn't in NATO, but they're an ally.
Besides, why would we allow countries in South America or Africa to join? This particular alliance is about thwarting Russian (previously Soviet) aggression. We can have other alliances with other purposes. For example, Japan isn't in NATO, but they're an ally.