I now that this subject is recent and political enough to be rather touchy for some, so I'll try to keep it non-partial.
NATO is a tricky thing to understand, as it has arguably seen more growth and activity after the end of the Cold War than during it. Its' principle reason to exist, the Soviet Union, simply went away, with no massive nuclear strike, no invasion of Western Europe, just a rather anticlimactic tumble. And, of course, there are many people, back then and now, that conclude "Mission Accomplished. The Reds are gone, Europe is safe from communism, Western Europe is fairly united and chance of war between them is non-existent, we don't need America and Europe locked into this thing anymore". Those same people probably assumed that NATO was going to, in a similarly quiet way, break apart from lack of a clear mission statement. Instead, of course, we've seen expansion in membership since 1991, including much of the former Soviet sphere, leading up to the fairly tense situation with Russia right now.
Suppose that was what indeed did happen. How would it occur? What would the repercussions be? Would alternative alliances come into being?