National needs for self-defence are claimed by all participants in the world arms race, from the superpowers to the smallest developing country ordering its armaments from European and American suppliers. Many small states maintain an unjustifiable amount of military hardware.
In the case of the USA and NATO, the policy of mutually assured destruction (MAD), and disavowal of the first use of non-conventional weapons, has now passed to a 'flexible response' justification for using nuclear and chemical weapons first, and to open contemplation of 'first-strike' capability as a defence option.
Classically, an arms build up would have been evidence of aggressive intentions, but modern military defence policies are built on tortuous reasoning and political pressures. Arms build-ups in the name of defence inevitably lead, first, to speculation concerning preemptive strike capability, and secondly, to the temptation to use what were once defensive weapons, offensively.