If you delve deeper into the history of NATO, you will realize that the threat that was most critical in the mind of some members and a key reason it exists is the threat of...Germany (who was not, initially, a member, even West Germany) becoming resurgent again once the allied occupation inevitably ended. (Which, ironically, ended up taking much longer than anyone would have expected because of the Cold War.)
NATO was flexible for these purposes because it was not defined or structured around any particular threat, but as a general-purpose regional security organization with a mutual defense commitment adaptable to changing threat circumstances.
I'm not entirely sure that that is the case and Germany was the primary reason for NATO forming. It surely was meant to hinder the spread of communism/Soviets.
And yes, a defense alliance can be flexible and change its objectives with the times, that's not counter to the point I was making. I'm not sure how to better express myself, I really think it is obvious that a defense alliance must be defending its members from _external_ threats.
If there is no possible threat (real or perceived, present or future) then no alliance is formed. That is the whole purpose, _the definition_ of a defense alliance.
I don't know how to express this notion with more clarity.
NATO was flexible for these purposes because it was not defined or structured around any particular threat, but as a general-purpose regional security organization with a mutual defense commitment adaptable to changing threat circumstances.