If you do not partake in a war when a war is waged against you, you lose, and you either get subdued, or perish altogether. This is why pacifism for some part of a society is only possible when another part of the society is willing and capable of using lethal force to defend the society as a whole.
Due to this, it's important to always have sufficient quantities of very efficient weapons, exactly so that you would never have to put them to use.