First : Certainly Islam is a problem. Just have look to some Koran sourates. Second: Bad migration management. Mitterand, the french socialist president and his prime Minister Mr Jospin , 25 years ago, began to leave France open door to immigration from Magreb. Algeria had millions of youngs unemployed at this time.
Then millions of them came not only workers but also all their families as the french industry employers wanted to stabilize these workers in our cowntry. Unbelievable but True! they were ignoring the huge social cost and future problems to come.
Since then, the FN and the PC alerted the danger, nobody wanted to hear them without shouting and insulting these peoples as racists and nothing changed during all following presidencies and still now!
These foreign families kept comming in and concentrated mainly around big town. They gave birth to Millions of children, birth rate was boosted by a very interesting and generous social system.
Nowadays our terrorists are almong these children.
They usualy have a lot of difficulties to live in our occidental society. Most of them, not all, are unemployed, they hardly write french, and they don't want to do their parent's job, many reject our way of life. All of that gives rise to terrorism.
Islam is the ideology to sustain that. Also some external islamist forces try to take advantage of this french bad situation. Hollande went to war abroad because of this internal french society problem but it's not the solution .
The solution is not outside it's inside. All european cowntries are more or less in the same situation.