armed citizens
Terrorism needs three things in order to produce victims.
Some disgruntled/deranged individuals to perpetrate the actual crimes.
Some callous individuals who for various reasons organize/support the disgruntled/deranged.
A large enough section of the community which is too tired/despondent/discouraged to care about what’s going on in its close vicinity – that’s where the terrorists (hit men and the support network) hide while preparing a hit and where the support network will try to sink itself afterwards.
Since it is practically impossible to corral all the deranged and to smoke out all the schemers beforehand the only really viable strategy  remains to make it so that the general public no longer assists catatonically to whatever is happening in its close vicinity. Until it’s so late that even drawing a gun is no longer very helpful.

I don’t even think that arms are such a must.
They might come handy in certain occasions but what we really need is a much more active attitude. A calm and considerate one but firm enough to impose respect.