Five million Syrians have fled their country over the past five years. Turkey has taken in 2.5 million; Lebanon, with a population of 4 million, has welcomed a million Syrians refugees. Last year the U.S. admitted 12,000 Syrians — while Canada took in more than twice that number.

Now Mr. Trump declares that no more Syrian refugees will be brought to the United States. And he has, by the stroke of a pen, cut President Obama’s commitment of 110,000 refugee admissions this year to just 50,000 — the lowest number in a decade. All this at a time when the number of persons displaced by violence and conflict are at levels not seen since World War II.

Trump’s Executive Order includes a statement that takes one’s breath away: “I hereby proclaim that the entry of nationals of Syria as refugees is detrimental to the interests of the United States.”

As a technical legal matter, terrorists and anyone remotely linked to a terrorist group are not eligible for refugee status. So on what basis could be that admitting Syrians “as refugees” can be contrary to American interests? One can argue about numbers, absorptive capacity, the responsibility of other states that might set some kind of limit on the number of refugees that the U.S. should accept. But for a President to declare that the admission of refugees can be “detrimental” to the interests of the U.S. is a statement so at odds with American traditions and values that one hardly knows where to begin in responding.

Many have quickly and powerfully taken up the cause, pressing arguments that ought, in a world that cared about sound policy, to bury the Trump position: the vetting of refugees is already more robust than for any other class of immigrants; the chances that a refugee will participate in or support terrorism in the U.S. are vanishingly small; the Trump policies assist ISIS recruitment efforts; our neighbor to the North shows how Syrian refugees can be admitted in large numbers without sacrificing security concerns; the Trump actions will serve as a model to other states and thereby undermine refugee protection around the world.

Similar arguments were vociferously asserted in the days following Trump’s announcement during the presidential campaign that he would ban all Muslims (refugees and immigrants) from entering the United States. Those criticisms led Trump to shift to a policy of “extreme vetting” rather than an outright, permanent ban. But the result could well end up the same if the Administration determines that no set of procedures can adequately protect the U.S. from infiltration by “radical Islamic terrorists.”

The direct harms and implications of Mr. Trump’s Executive Order are dramatic, both in the near and long term. As an immediate matter, tens of thousands of persons who have fled violence and conflict in Syria, who have given up homes and possessions to save themselves and their families, who have seen their communities destroyed and have now spent up to five years in inadequate shelter and with no job prospects, whose children are now referred to as “the lost generation” because of the years of schooling they have missed — these are the people that Trump’s order will hurt.

It is true that refugees now residing in Turkey, Lebanon and Jordan may not be facing immediate return to the conflict in Syria; but they are experiencing a “second exile.” Their exclusion from economic opportunities and social programs renders them unable to rebuild their lives and their communities. This is the reason for refugee resettlement programs — to enable people whose lives have been on hold for years to begin to return to normalcy. (Mr. Trump’s policies not only stop refugee admissions from Muslim countries; he has cut overall admissions by more than half. So his actions will reverberate in refugee camps and settlements around the world.)

An additional, crucial reason for refugee resettlement is to help share the responsibility for refugee protection. States neighboring countries in conflict bear the burden of refugee flows. Peter Sutherland, the Secretary General’s Special Representative on International Migration has labeled this “responsibility by proximity.” The vast majority of the world’s refugees are located in the developing world, in countries that are often unable to provide adequately for the basic needs of their own citizens. The global system is obviously not working fairly if those most able to assist don’t do their part. Hosting states at some point will simply close their borders to refugees — as Lebanon, Jordan and Turkey have now largely done, after accepting more than 4 million Syrians. And they will be unable to provide adequate care for those they admit.

The human and systemic costs of banning refugee admissions to the U.S. is thus clear. What, then, can account for the Trump policies? What American interests could “outweigh” the humanitarian ends to which the U.S. has been so long committed?

The Trump Executive Order is purportedly all about security — although this is somewhat undercut by the fact that its suspension of immigrant and non-immigrant (tourist, student, business visitor) visas applies to six Muslim nations but not to Saudi Arabia, Pakistan, France and other countries whose citizens have committed terrorist acts. The Order states that refugee admissions might resume again if additional procedures are put in place that “are adequate to ensure the security and welfare of the United States.” The standard that will be applied is not specified. It is possible that the Administration will say that given the risks, only 100% certainty will do. But a standard of certainty, or near certainty, that harm has been prevented is of course unattainable. Nor is it sensible. It is not one we apply to cars or guns — both of which impose dramatically greater risk to Americans than do refugee admissions. Indeed, if the standard were to be applied to all persons seeking entry to the United States — as tourists, students, business visitors, family members — it would effectively end immigration to the U.S.

Perhaps a (near) certainty standard will in fact be adopted as a means to putting the refugee program on indefinite hold. But if this occurs, it will not be based on sound or consistent policies on security. Rather, ending refugee admissions would be a cover for even more troubling ideological grounds.

One such ground would be Mr. Trump’s apparent view that we are fundamentally at war with Islam. His extreme statements in the past depict the religion as motivated by hatred toward the West and dedicated to inflicting the greatest harm possible on the United States. Indeed, he brushes off concerns that banning Muslims would drive new recruits to ISIS: they already hate us and nothing we do will change that except eliminating them.

That these new policies are primarily directed at Islam appears on the face of the Executive Order, which permits the entry of refugees even during the period of suspension, if “the person is a religious minority in his or her country of nationality facing religious persecution.” There can be no doubt that this provision is included to permit the entry of Christian refugees while maintaining the ban on Muslims.

This religious discrimination is flatly prohibited by the international Refugee Convention and subject to constitutional challenge. It is not clear how much that will matter to the Administration. The ill-fit of the ban on security grounds — that it leaves out some Muslim states whose citizens are terrorists — can be used by Administration lawyers to defend the ban against a charge of unconstitutional religious discrimination. But this may have been strategic only. Recall that the Bush Administration, in establishing its post-9/11 registration program in the United States included a large number of Muslim countries and also North Korea.

A different ideological reading of the Executive Order would place it within Trump’s America First nationalism. From this perspective, the physical wall against Mexico is of a piece with the bureaucratic wall against refugees. Both are directed at “foreign enemies”: terrorists and “illegal aliens” (or in Trump’s inelegant terminology “criminals, drug dealers, rapists”) against whom a white Christian nation must not let down its guard. (According to Trump, “illegal aliens” have even invaded the U.S. political system, by casting millions of votes in the presidential election.)

There is an obvious counter-narrative here. Based in humanitarianism, it declares (to paraphrase a Catholic aphorism) that we help these vulnerable, needy people “not because they are Christian, but because we are Christian.” This is the narrative that has sustained refugee admissions for decades and in all likelihood is shared by more Americans than either Trump’s anti-Islam or race-based nationalism. And it is possible that with a powerful enough public outcry, the Administration will change course.

It has left itself an out to do so. It would happen this way: in a few months, the President would make a finding that new procedures put in place by his Administration adequately protect America and thus refugee admissions can resume (this of course could be done simply by affirming the current tough clearance procedures). This allows the President to be both strong and generous, protecting the American people and allowing refugees in once all is safe.

The President may, however, decide to stick to his guns. He can take the view that America cannot be safe until there is no ISIS fighter left who could sneak into the country through the refugee program. And he can find that no set of screening procedures can provide the level of certainty that the threat demands. That is, the admission of Syrian refugees would remain “detrimental” to the interests of the United States. This would leave the U.S. in a state of continuing war against Islam, with refugees as the chief victims — a tragic irony for those who have already been victimized by the conflict in Syria. Indeed the inability to come up with fool-proof screening procedures might then “justify” broad measures against the domestic Muslim population, since we will never be able to be sure who is for us and who is against us. At that point, we will have lost, as a nation, far more than our refugee admission program.

We have gone through these anti-foreigner spasms before: the internment of Japanese Americans, the Chinese Exclusion laws, the National Origins Quota system, Operation “Wetback.” One would think that our national shame over these laws and executive actions would have been a bulwark against this disgraceful Executive Order. But not, apparently, in the age of Trump.

Mr. Trump’s Executive Order inflicts injury on tens of thousands of innocent people. And it harms all Americans by diminishing the moral standing of the nation in the eyes of the world.