For neo-realism, the presence of a common threat and a bipolar power structure is imperative for the existence of NATO. The end of the Cold War resulted in a unipolar world order which, according to neorealists, does not justify the existence of NATO. NATO “could persist on paper but it would cease to function as an alliance. Waltz wrote in 1993 that "NATO's days are not numbered, but its years are." However, the sustainability of NATO has led some of them to review their positions. Waltz argued that NATO can no longer be considered the same entity because it has changed significantly since the end of the Cold War. However, Waltz acknowledged that "it is the old NATO objective that has disappeared, not NATO ».