To add some more context, NATO's refusal to establish a NFU policy dates to the Cold War, when NATO military leaders concluded that defending central Europe from Soviet invasion required a credible threat to deploy tactical nuclear weapons. Confronted with that threat, Soviet forces would have spread out to avoid concentrations large enough to be vulnerable, making a conventional defense feasible by preventing the outnumbering Soviet army from massing to take defended positions, which in turn would provide time for the USA to deploy large armies into Europe. Without this credible threat, the Soviet army would have had the option to pick off a NATO country (i.e. Western Germany) by massing armor columns and forcing a surrender before the USA could deploy a comparable army across the Atlantic.

NATO no longer faces this threat, but the policy remains.