From the Vox article noted by Steve Sailer earlier:

So it’s natural to conclude that if we could just keep dangerous outsiders from getting into our countries, we would be safe.

Natural, but wrong.

Europe's terrorist threat, while rising, has come overwhelmingly not from foreigners but from fellow Europeans. And terrorism experts say it's Europeans who've traveled to fight with ISIS and are now coming home, not refugees or migrants, who pose the real threat.

Closing borders and cracking down on refugees won’t make us safer, because that's simply not where the problem comes from — and in the process of not protecting us, it will also hurt innocent people.[Emphasis added]

Of course, none of these Europeans are actually Europeans—Vox's "fellow Europeans" are Muslim Arabs born in Arab suburbs of Paris or Brussels. On Twitter, Ross Douthat of the NYT, where he's the token sort-of-conservative asks:

Um ... so how exactly did Europe end up with so many "citizens" who despise European civilization? https://t.co/2cPMAGLkVh

— Ross Douthat (@DouthatNYT) March 23, 2016

Various Twitter answers:

Can't load tweet https://twitter.com/TiloJung/status/712634061693575170: Sorry, that page does not exist

Can't load tweet https://twitter.com/mattyglesias/status/712662883776262144: Sorry, that page does not exist

@douthatnyt Europe had those. Anarchists, socialists, fascists, & modern-day totalitarians of every sort. The issue is not who you let in.

— Austen Ivereigh (@austeni) March 23, 2016

None of them appear to get it, or they're just putting their fingers in their ears and shouting "La, La, La, I can't hear you!" After the Paris attacks, Ann Coulter wrote a column we  titled No "Frenchmen" Or "Belgians" Committed Paris Attacks—Only Muslim Arabs.

But there is one point where the Vox people are right: Europe's Muslim problem can't be solved just by closing the borders Muslim immigration—which is why Peter Brimelow's post-San Bernardino  column (reprinted yesterday ) was titled San Bernardino: The Answer Is An Immigration Moratorium—And Muslim Expulsion.

UPDATE: From the NYT: "The brothers — Ibrahim el-Bakraoui, 29, and Khalid el-Bakraoui, 27 — were Belgian and had criminal records, officials said. But the pair had no known links to terrorism..." except for, you know, being named Ibrahim and Khalid, like no actual Belgians ever.