That is a blanket statement which is applicable in some cases and is not being applicable in the others. You can say that about almost anything and twist it the way you want.
Example:
Ivan has a gun/knife/stick, so nobody attacks him for no reason. The possible consequences are: (1) Everybody should carry a gun/knife/stick to feel safe. (2) If Ivan was being attacked, that was for a valid reason. (3) If you see see anybody with a gun/knife/stick, you should strike first with a better gun/knife/stick.
Everybody who attacks usually has reasons. If you are a country which is attacking another country with nuclear weapon, you have to first think how to neutralize that weapon or make sure the weapon won't reach you. Once the machine gun was percieved as a war stopper. Indeed, why to attack if the [especially fortified] defense could kill every approaching soldier in the range?
There's no better weapon or worse weapon or obsolete weapon. Every weapon kills. The fight is about the right weapon for the right situation and about creating situations that would benefit your weapon and disadvantage your enemy's weapon. The nuclear weapon is not an absolute weapon and not an exception to that rule. You can attack a country with nuclear weapon and win.