Fact One: The threat that is presented to justify President Trump's executive order on immigration is 99.99 percent imaginary. All studies of violent terror attacks in the United States since 2001 show that the overwhelming majority were committed by native-born or naturalized U.S. citizens or permanent residents and not by persons who were admitted as refugees.
Exact statistics vary a bit in different analyses because they don't use precisely the same definitions, on what, for example, is a terrorist act. But the numbers in all the studies are negligible, from three or four up to a dozen or so cases out of the more than three-quarters of a million refugees, about a third of them Muslims, who have arrived in this country since the attacks on 9/11.
In that already tiny number, almost none resemble the scenario promoted by those who favor more restrictions: radical extremists sneaking through the security screen and reaching this country with the intent of committing terrorist violence.
Depending on what definitions are used, no more than one or two cases come close to meeting that description. By some standards (counting only fatal attacks, for example) the number would be zero. That's for all refugees, not just those from the seven countries whose citizens in all categories, not just refugees, are banned under the executive order.
As one analysis found, not a single American has been killed in the United States by a refugee from any of those countries.
To sum up: the threat of jihadist terror in the United States has not come from Muslims who were radicalized overseas and got visas so they could come here and commit violence. Virtually without exception, the threat has been from people who were radicalized in the United States.
No new immigration control or visa screening will protect against that threat. Counter-terrorism professionals overwhelmingly agree that the best defense is when national and local law enforcement and other public institutions have enough trust and strong enough relationships in the Muslim community so members of that community will come forward and tell the authorities when they see someone showing signs of dangerous attitudes and behavior. This is a judgment, not a fact, but it is hard to see how the Trump administration's immigration order moves anywhere but in exactly the opposite direction.
Fact Two: A core premise of the jihadist extremists is that America is the enemy of their religion and that Muslims have to defend their faith by violence. Beyond possible doubt, as a direct result of the Trump immigration order, that message is more credible today with more people around the world than it was a week ago.
Denials from Washington will not change that. Americans may choose to believe Mr. Trump and his supporters that the order was not directed at Muslims' religion, or they may be skeptical.
But it is delusional not to recognize that most people in the Muslim world will undoubtedly see themselves cast as America's enemy. That validates the jihadist narrative and weakens our efforts to work with Muslim countries against the dangerous forces in their societies.
We will never be able to quantify exactly how many more fighters the Islamic State will enlist as a direct result of Mr. Trump's action, but there is no possible question that it will help their recruiting. Nor will we be able to count how many fewer Muslims will join the anti-extremist side, but the number will certainly be lower than it would have been without this policy. Without them the fight against the extremists will be more difficult, not easier.
Since the number of Muslims in America who will trust and cooperate with law enforcement and other public agencies to help find and head off possible violent radicals will be lower, too, Mr. Trump's directive will not make us safer either at home or overseas. It will make us significantly less safe.
Fact Three: It's not just logic that tells us that making whole countries and communities targets of suspicion will not help protect us from terrorism. We have learned that from actual experience. In the aftermath of 9/11 — even while President George W. Bush, unlike President Trump, strongly assured Muslims that they and their religion were not our enemies — various U.S. counter-terror agencies attempted to detect possible plots by targeting large Muslim populations in this country. Those programs did not work.
One prominent example was the National Security Entry-Exit Registration System (NSEERS), which applied to "non-immigrant" men over 16 — men who were in the United States but without permanent residence status — from 24 predominantly Muslim countries plus North Korea.
Between September 2002 and September 2003 NSEERS registered more than 83,000 men. More than 13,000 were issued deportation orders, almost all for overstaying their visas or other immigration violations. But from all available information, the program did not lead to a single terrorism-related conviction.
Similarly, six years of spying by a secret unit of the New York Police Department on mosques, Muslim student and community organizations, and other Muslim targets did not turn up any terrorists either. Other such programs had the same lack of result.
Successful terrorism investigations, by contrast, often resulted from tips and cooperation from within the Muslim community. The lesson is that to protect against terrorism, America needs to make allies, not enemies, in the Muslim population, here and abroad. President Trump's immigration policy does the opposite. It puts us in more danger, not less. Americans — including members of his party in Congress and counter-terror and national security professionals — need to recognize that and do everything possible to show the world that this ill-conceived action does not represent American principles or beliefs.
Arnold R. Isaacs is a journalist and writer. He is the author of From Troubled Lands: Listening to Pakistani and Afghan Americans in post-9/11 America, and two books relating to the Vietnam war. Mr. Isaacs provides a selection of studies that have analyzed the connections between immigration status and terrorist events in the United States.