Trump’s travel ban is useless. Terrorists come mostly from our own back yard.

Shortly after the news broke Saturday evening about a possible terrorist attack in London, President Donald Trump tweeted: “We need to be smart, vigilant and tough. We need the courts to give us back our rights. We need the Travel Ban as an extra level of safety!”

A good deal of the judicial opposition to the Trump administration’s proposed temporary ban of citizens of six Muslim-majority countries from entering the United States has focused on the constitutionality of banning adherents of a particular religion. It makes sense, given Trump’s many public statements during the campaign about barring Muslims from coming to the States.

Less attention has focused on the question of whether the travel ban would do what it is intended to do, which is purportedly to make us safer. The reality is that it probably wouldn’t do much to protect Americans and Europeans from the kind of terrorism we mostly witness now.

In a large majority of recent terrorist attacks in the West, the attackers have been native-born citizens rather than recent immigrants or refugees.

The identities of the London attackers have not yet been released, but it’s not unreasonable to assume they probably are British citizens - after all, almost all the perpetrators of serious attacks in the United Kingdom over the past decade or so have been British citizens.

Three of the four suicide attackers recruited by al-Qaida who carried out the most lethal terrorist attack in British history, killing 52 commuters on the London transportation system on July 7, 2005, were British citizens. So, too, was the suicide bomber who carried out an attack in Manchester two weeks ago that killed 22 people attending an Ariana Grande concert.

The same pattern holds true in the United States, according to research by New America, a Washington, D.C.-based think tank of which I am vice president. Of the 13 perpetrators of lethal jihadist terrorist attacks since 9/11 (which produced a combined fatality count of 94 people), all were American citizens or legal permanent residents. Of the 406 cases of jihadist terrorism (nonlethal and otherwise) since Sept. 11, 2001, more than 80 percent involved U.S. citizens or legal permanent residents.

None of the lethal terrorists were refugees, nor were any of them from any of the six countries the Trump administration would like to suspend travel from, nor were their families from any of those countries, and only one was a relatively recent immigrant (from Pakistan, which is not on the travel ban list). Instead, many of these attackers were radicalized, at least in part, by materials they read on the Internet, or through communications with other jihadist militants on the Internet.

The travel ban would, of course, be of no use whatsoever in blocking the Internet.

On Sunday, British Prime Minister Theresa May took a swipe at social media companies declaring, “We cannot allow this ideology the safe space it needs to breed. Yet that is precisely what the Internet - and the big companies that provide internet-based services - provide. We need to work with allied, democratic governments to reach international agreements that regulate cyberspace to prevent the spread of extremism.”

That is easier said than done. All key social media companies are based in the United States, and the First Amendment allows for much hateful but protected free speech. And when it comes to terrorism, the line between what is admissible on a social media platform and what is not are much fuzzier than in cases such as child pornography, which is both illegal and easy to recognize.

Trump is unlikely to be granted his travel ban, given the number of courts that have ruled against it, but in the event that it is enacted, he will find that it is far from the magic bullet he believes it to be.

Instead, the best approach to deal with the scourge of jihadist terrorism is to enlist rather than alienate Muslim communities, because it is often peers and family members who are best positioned to notice radicalization or attack planning. Indeed, Muslims already are leading anti-radicalization efforts in their own communities, and we should support these ongoing efforts.

But not even that is a panacea. Members of the Muslim community warned authorities about the radicalization of the Manchester terrorist Salman Abedi years ago, and his father was so concerned about his son’s state of mind that he had confiscated his passport before the Manchester bombing.

None of this, of course, was sufficient to deter Abedi from his deadly attack.

Bergen is the author of “United States of Jihad: Investigating America’s Homegrown Terrorists,” a CNN national security analyst and vice president at New America.

This story was originally published June 6, 2017 at 5:40 AM with the headline "Trump’s travel ban is useless. Terrorists come mostly from our own back yard.."