In decades past, you’ve seen home grown militant groups in many EU countries, Germany has been no exception back in the 70s and early 80s.
In places like Ireland, you might start to see descendants of the IRA and other violent Irish groups come back to life.
It won’t be all out war, it will be a Mosque getting set on fire, followed by endless riots and counter protests happening on a regular basis.
Guerilla warfare, deaths, and stalemates. Unfortunately, in the long run, contraception and abortion favors the savages.