The premises of Mr. Trump’s Executive Orders on immigration enforcement are simply stated. Persons not authorized to enter the United States should be stopped at the border. Those who are here in violation of law should be sent home. The federal government should be able to seek and accept state and local government assistance in removing undocumented migrants. Stated at this level of abstraction, most Americans would agree.

The problem, of course, is how to move from premises to policies, and the particular problem for Mr. Trump is a fundamental misunderstanding of the current facts on the ground. Let’s start with the facts.

Since the beginning of his campaign for President, Mr. Trump has painted a picture of a border out of control, with Mexico sending “criminals, drug dealers, rapists” across the border to the U.S. One would have thought, listening to him, that undocumented migration was on the rise, that a feckless federal government was either unable or unwilling to stop the flow, and that immigrants convicted of crime were permitted to roam free in the U.S. to commit more crimes rather than being deported as required by law.

But even the merest regard for the facts show these claims to be canards. Due in large part to a massive build-up of enforcement resources at the border over the past several decades (including a quadrupling of the Border Patrol since the beginning of the Clinton Administration) the undocumented flow has decreased dramatically. As has been widely reported, apprehensions at the southwest border have plummeted by nearly 75% since 2000 and deportations have sky-rocketed. All told, there were more than 3 million removals during the Obama Administration (immigration advocates applied to Barack Obama the stinging label of Deporter-in-Chief). Perhaps most startling — and what no doubt would be news to Mr. Trump — is that in recent years the United States has witnessed a net outmigration of Mexican nationals; that is, from 2009 to 2014, more Mexicans left the U.S. than entered.

Perhaps Mr. Trump believes the statements he makes, much as he apparently believes — without any evidence — that 3 million “illegal aliens” voted in the presidential election. Or perhaps the incendiary comments were cynical appeals to voters open to such fear-mongering. But either way, he has now made his baseless assertions the basis of a federal enforcement policy that will harm families, destabilize communities, and stigmatize persons based on their ethnicity (immigrants and citizens alike).

Over the past several decades, U.S. immigration policy had arrived at a rough accommodation that had pursued goals of toughness and fairness in about equal measure. Indeed, until Mr. Trump played the immigration card, immigration was not a major issue in the campaign.

A crucial element, as already mentioned, was enhanced border enforcement. Congress had demanded a secure border, and the Executive Branch largely delivered one through a combination of barriers, more personnel (strategically deployed), and hi-tech sensors. The idea of building a continuous wall was never seriously considered for a host of reasons — from the topographical and environmental to respect for private property and cost-effectiveness.

Second, it was recognized that removal of 11 million undocumented migrants inside the U.S. was neither feasible nor humane. In a world of limited resources, priorities had to be adopted; and the Obama Administration, continuing policies of previous Republican and Democratic administrations, sensibly put first the removal of immigrants convicted of crimes. Other categories of undocumented migrants might ultimately have legal grounds to remain, based on family, work or an asylum claim; so making those cases a lower priority also made sense.

Third, the federal government could seek but not command or coerce state and local assistance in immigration enforcement. Principles of American federalism supported local choice. Some cities and counties adopted “sanctuary” policies that prohibited their officials from cooperating with federal immigration enforcement efforts (based on the view that immigrants were less likely to report crimes if they thought their status would also be investigated). Others signed agreements to cooperate with the federal government in the removal of migrants convicted of serious crimes.

Fourth, polls show that two-thirds of Americans support “a path to citizenship” for undocumented migrants who have resided in the U.S. for an extended period, paid their taxes, and not committed a serious crime. The undocumented population is, viewed rightly, neither as a security threat nor a threat to American jobs. Even accepting Mr. Trump’s counterfactual claim that the border is out of control, it is interesting that the unemployment rate of U.S. workers is now at the lowest level since 2007. And there is widespread support for Mr. Obama’s Executive Order granting lawful status to DREAMERS, undocumented migrants who came to the U.S. as children and are attending school or have graduated from high school.

Last, the accommodation included procedures at the border that permitted “expedited” return of persons without papers while preserving a route for persons fleeing harm in their home countries to file for asylum. In the face of a significant increase of migrants from El Salvador, Honduras, and Guatemala (the “Northern Triangle”), the Obama Administration invoked a harsh policy of detention that put families behind bars in an attempt to deter additional flows. That was stopped in the courts, leading DHS to more sensible rules for release of asylum-seekers who demonstrate a “credible fear” that they will be harmed if returned home.

Not everyone was happy with this congeries of policies. Some on the right thought that it titled too much toward the immigrant and was not tough enough on “sanctuary cities”; some on the left strenuously objected to the increase in deportations (arguing that the category of “criminal alien” should not include persons convicted of minor offenses and misdemeanors) and thought the Obama Administration did not push hard enough for “comprehensive immigration reform.”

Furthermore, the immigration system faced other serious problems that were being ignored. The asylum-system is under extreme pressure, with applicants waiting years for decisions. The enforcement of employer sanctions (fines for employers who hire undocumented workers) has virtually stopped. Immigration judges render thousands of removal orders; but unless the alien is detained, he or she faces little likelihood of being picked up and returned home.

But in the main, America had worked out a way of understanding immigration that played to deep American values of respect for the law, fairness, federalism, and humanity. And it was moving toward a compromise that would bring the large undocumented population out of the shadows.

Now comes Mr. Trump. With slogans and bluster, he says he will fix a system that, while under pressure, in fact is not broken. His program is one of punishment for “illegal aliens” and their families, and for cities that dare to view them as members of their communities. A policy of mandatory detention of all removable persons is not feasible and will burden thousands of bona fide asylum-seekers. On top of all this, it will be hugely expensive — the wall, 5,000 more border patrol agents, 10,000 new ICE officers. If the wall is built, and it far from clear that it will be, it will be a monument not to sound policy but to ego and showmanship.

Mr. Trump’s Executive Order hyperbolically states that sanctuary cities “have caused immeasurable harm to the American people and to the very fabric of our Republic.” How long will it be before those words are applied to Mr. Trump’s new policies?