The mistake France made is they let people go to Syria to fight with ISIL and then let them came back to France. Their "solution" was to just keep an eye on the returnees. For example, they were watching the brothers who did the Charlie Hebdo attack, but stopped the surveillance six month before the attack to watch other people.

We try to arrest and imprison people BEFORE they get to Syria. Maybe France should go pick up the 200 or so known returnees and see what they have been doing. It might be interesting to search their spaces to see if they have any guns, tap their phones, track them for a few weeks to see if they go meet with attack planners, etc.

Hopefully France does't do something big and stupid with their many hydrogen bombs. They could turn all of Syria into a plain of molten glass, if they wanted.