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.