When I was taking a taxi to the airport in Las Vegas after Freedomfest last month, the driver and I got to talking about Donald Trump who had blurted out a series of incoherent non-sequiturs at the conference. It turned out that the driver was fan of Trump, especially his promise to build a fence to keep out illegal immigrants. The driver was under the impression that immigrants committed a lot of crime, e.g., Trump's criminals and rapists flooding across the border. I tried gently to point out that the data do not support those claims.
Well, retorted the driver, we must close down the border in this "time of terror." Trump also apparently believes that Islamic terrorists are creeping across the Mexican border to blow up stuff and behead people. (See video below).
I didn't have the data to hand, but I tried to think of any post-9/11 attacks that had been launched by terrorists who had sneaked across our borders. Well, I have checked around a bit and it turns out that all of the attacks even vaguely motivated by Islamist ideology have been by citizens, green-card holders, student visa admittees, and so forth. (For more detail see this fairly comprehensive list of attacks compiled by the Heritage Foundation.) It is true that some attackers did overstay their visas, but all were legally admitted across our borders. In other words, building a wall across our border with Mexico would have done nothing to prevent any Islamic terrorist attacks.
Keep that mind when you listen to the border security bloviations during the Republican presidential candidates debate tonight.