But ruling isn’t the only possible goal. You could intend to destroy the military/economic/social structure of another people. Terrorism is a cheap way of degrading these things. Every extra security guard you hire, and every set of metal detectors and explosive sniffers, represents money you can’t spend on things people need. When you can trust people you don’t have to build in the reinforcements. Travel doesn’t have to demand extra hours going through checkpoints and searches when people can be, generally, trusted. Soldiers cost a lot, but so do guards. There seems to be an irreducible minimum of predators (O(2%) who are dangerous?) in any society, but you can always create or import more.
I don’t think Muhammadan migrants are going to start ruling France anytime in the next century or so. But I do think they can (the imported labor from earlier decades already does) degrade the trust in the society and raise the unproductive expenses higher and higher. After a while the margin runs out and the real productivity drops and social fabric unravels. After a while the country is poorer and has less capacity to build or to trust. The first trust to go is between the natives and the newcomers, followed quickly by that between the natives and the old elite. But it doesn’t stop there; the new elite aren't generally terribly trusting of each other either. The circle of trust shrinks to those you know.
Without trust you don’t have a nation. An empire, maybe, but not something people will risk themselves for.
FWIW, I agree with David Warren that angry natives will typically attack nearby newcomers (a sample enriched in those trying to assimilate), rather than risking the strongholds of the real trouble-makers or doing the painstaking research to find out who are the hidden foes. This will have a readily predictable effect.