By making the secret services work for their money on real problems, instead of fretting about tapping media pirates and suchlike. By having a foreign policy that does not rely on military intervention at the drop of a hat. By not starting fires all over the place. The list is long and well-known.
>>toyg+(OP)
I don't think any secret service could prevent any isolated individual to make at home, say, Molotov cocktails, and throw them by car in a crowd.
>>S4M+O1
But these weren't isolated individuals. This was an organised network with grenades and assault rifles and the training to handle them.
This is not a Breivik, or a "Shoe Bomber" Reid; this is '70s-style, organised, cross-border terrorism -- the sort of which "we" were supposed to be good at handling by now.