Generally speaking, the only major effect actually accomplished by terrorists is to create a pronounced backlash.  The backlash tends to be part of a vicious circle, in that it fosters acting out by more people whose tendency to commit terrorist acts would otherwise remain latent.

How Terrorism Can Alter Elections

More details on the attacker, Karim Cheurfi.  He had previously been in Algeria, earlier in January and February.  He also had an offense in 2001 for firing at police officers, for which he was sentenced to 15 years in custody.  Self-proclaimed Islamic State, obviously, did not exist in 2001, so it is absurd to say that IS indoctrinated or created Karim.  He adopted the cause of IS.

Champs Elysées, fermati 3 familiari dell’attentatore. Accanto al cadavere trovato messaggio di sostegno a Isis

Xavier, il poliziotto ucciso agli Champs-Élysées: 37 anni, attivista gay, era alla riapertura del Bataclan