I think it is very telling that the destruction of this monument was attached to ISIS. It may even be that ISIS was not directly responsible,but the fact that the organization was implemented in this act indicates that either they have a strong foothold in Palmyra, or else they plan to soon.
Isis has very strong foothold. They have worked hard to prey on the fears of the people in Palymyra. These people are very simple, traditional people who are afraid of change and the growing influence of the western world. It is easy to exploit the fear and hatetred of people like this.
Yes, ISIS has a strong foothold in Palmyra, because they were table to destroy the monument without repercussions. They wouldn't have been able to do that in any other place without being arrested. Destroying public property is a very serious offense and they must have a stronghold in order to be able to do stuff like that.
No, they're opportunist thugs who go around destroying whatever they can. They don't have a strong foothold anywhere. They just create chaos and destruction as they blaze forward in anger and violence. They will destroy whatever they encounter, and one day they'll destroy themselves. They have no organized plan. In the end they will lose.