NATO was created as a DEFENSIVE collective security alliance for the purpose of deterring any Soviet attempt to take more territory in Europe than they had been able to gain control of in the years immediately following WW2. By the time NATO was created the Soviets had nuclear weapons and the emphasis in NATO was always on deterring the Soviets from an advance into Western Europe by creating the impression that any such advance would cause a conventional war in which after a massive and very destructive struggle NATO would probably be defeated and would likely resort to tactical nuclear weapons in an exchange that would escalate to general war and Armageddon. To make this strategy work it was necessary that the conventional forces committed to Allied Command Europe (ACE) were sufficiently potent to make it clear that conventional Soviet forces would not have an easy and quick victory in which a "done deal" would be accepted as not worth mutual annihilation. Does that sound familiar? During the Cold War the process of keeping the NATO forces strong enough to make the deterrence "work" was great fun for all involved, exercises, books, promotions in large forces, etc. There was also a hell of a lot of money to be made in building the equipment involved. When the Cold War ended NATO would logically have been dismantled, but, the Hegemonist Temptation gripped the elites in the West and the alliance was illogically driven eastward to the borders of Russia and Russian attempts to join this supposed mutual defense arrangement were rejected. This made it clear to the Russians that the alliance was an anti-Russian alliance.
Somehow the Europeans were persuaded that NATO should become an "out of theater" instrument of US power.
That seemed to be an acceptable situation when the USA and Russia were not on a collision course as we are now, but both the USA and Russia still possess their roughly symmetrical nuclear weapon armaments and MAD still apllies.