Let the first Mosque get blown up and it will be on, car bombs, assassinations, hostage situations, drive by shootings, etc...will become the norm.
I would imagine with the arms trade flooding Europe with tools of war, plenty is being siphoned off in the staging countries like Poland, etc. which will eventually make their way throughout Europe.
China, Iran, others.... etc. would be more than happy to flood Europe with arms in the event of a ‘civil war’. Qhadaffi was THE major arms supplier of the IRA in the 70’s.