But did NATO (ever in its history) function as a defence alliance, not as an agressive alliance, though?
Early in the cold war there were a few senior generals on both sides that thought the US could or should make a first strike against Russia. But the leadership and the populous on both sides of the curtain never thought that was what it was all about at all. The aggressor was always going to be Russia invading western Europe, and NATO was always about defending it.
But go ask random people on streets in the US, Russia or Europe, and they will focus on the "cold war" and the iron curtain and NATO's defense of the west against the threat of Soviet expansion.
The NATO treaty only protects members if they were attacked in Europe or the North Atlantic. If bin Laden had hit the Sear Tower instead of the WTC it would probably not have been a NATO operation.