Just to bring some data to the discussion:
Muslims in Europe.
Countries of western Europe with the highest muslim percent of the population:
France: 7.5% (4.7 million)
Netherlands: 6.0% (1 million)
Belgium: 5.9% (0.6 million)
Germany: 5.8% (4.8 million)
Britain: 4.8% (3.0 million)
A substantial portion of Dutch muslims are (or are descended from) former
gastarbeiters from Turkey (40%) or Morocco (35%). A substantial portion of French muslims are Arabs, especially north African muslims. A lot of British muslims are of Indian/Pakistani descent. These are due in part to former colonial status.
Counties with the greatest number of arrests for religiously-inspired terrorism:
France: 188
Britain: 132 (in fairness, the Brits do not record the apparent motivation of terrorists).
Belgium: 71
Germany: 17 (63.2% of German muslims are of Turkish descent)
Netherlands: 16 (38% of Dutch muslims are Turkish descent and 26% "Asian"
So, there may be a causal relationship between number of muslims and the number terrorist arrests, and the national descent of muslim immigrants might also explain something as well.