What used to be bitter slums are now militant colonies.

The Muslim guts of European cities, from Brussels to Marseilles, and the high-rise banlieues of Paris (designed by the left as earthly paradises) or the grubby Stadtteile of Cologne or Hamburg, are no longer way-stations for readjusting immigrants. They’re imperial holdings of a bloodthirsty caliphate, poisoned with the spirit of jihad.

And the Islamist fanatics who rule from the shadows intend to slaughter the trusting aborigines, whose tribal chiefs are enchanted by the trinkets of political correctness — as we were reminded, again, Tuesday when terrorists killed 31 and injured hundreds in a double-barrel attack in Brussels.

Will Islam really conquer Europe? Of course not. But reality leaves Islamist terrorists unconvinced and unmoved.

And they’ve already disrupted societies, upended politics and seized strategic ground at Europe’s heart: The Brussels attacks were symbolically perfect, striking not only a national capital, but the headquarters of the European Union and NATO.

Consider Belgium. The police and intelligence organizations (mindlessly divided between Flemish- and French-speakers) couldn’t crack the local Muslim community. Contributing scores of jihadis to ISIS in Syria and Iraq, Molenbeek and neighboring slums also protected the terrorists in their midst.

Some locals must have known. Many more must have suspected. But none warned the authorities that hundreds of their “fellow Belgians” faced impending attacks.

The roots of radicalism had gone so deep that support for the fanatics trumped even the desire for self-protection. Of course, the locals saw that, in the wake of multiple terror attacks and arrests, they’d face suspicion, scrutiny and an end to the craven tolerance of their crimes.

But nobody talked. Nobody will.

Nonetheless, “humanitarian” groups will continue to defend the “rights” to government financial, medical and housing support for those who send their sons on jihad and beat their daughters to death.

Which brings us to the question: What can Europeans do? If they somehow summon the strength of will? Before the situation worsens monstrously?

Apart from the obvious measure of restricting migration that seeks only to feed at the public trough, there are three steps Europeans could (but likely won’t) take:

First, work on the families. In the Islamic cultures of the Middle East and North Africa, the family is the fortress. European states need laws that permanently deny all further state assistance, including subsidized housing, to the extended families of terrorists and their abettors.

Collective punishment? Islamist terror is a collective crime. As the residents of Molenbeek just reminded us. You have to do what works. And we need to remember the elementary truth that receiving countries owe immigrants nothing beyond their physical safety. We’re not in their debt, they’re in ours.

Second, not only resident status, but citizenship must be revoked from anyone affiliated with terror — including birthright citizenship. We must stop letting left-wing activists creatively interpret international law to protect monsters bent on massacre. The first human right is for law-abiding citizens to be able to live in peace, free of bodily harm.

Third — and toughest — the Europeans need to find ways to break up the Islamist colonies that even the police prefer to avoid. Just as European colonialists brought smallpox to the natives of the Americas, the jihadi colonists bring their plague of fanaticism.

Even in our own country, where a different class of Muslim immigrant — largely educated, ambitious and law-abiding — means we don’t face the crisis Europe does, the toughest problems we do face come when Muslims from underdeveloped states are allowed to immigrate in mass and concentrate in one area.

That lets them cling to their old culture and language, instead of learning English and digesting our social values. Note the way we’ve mishandled Somalis, who, in turn, have disproportionately slipped off to jihad.

American Muslims are yet another of our nation’s success stories (though we need to confront the bad actors abusing our tolerance). We get genuine immigrants seeking their American dream. Europe gets fanatical colonizers.

Don’t think there’s a difference? Check out Brussels.

Ralph Peters is Fox News’ strategic analyst.