Protesters rally in opposition to President Trump's immigration order in New York on Jan. 29. (Kathy Willens/Associated Press)

IN THE fiscal year that ended last fall, the number of undocumented immigrants apprehended on the southwestern border was just a quarter the number in 2000 and less than half the annual count during most of George W. Bush’s administration. Although last year’s apprehensions in the Southwest rose from the previous year — largely because of unaccompanied minors and families from Central America seeking refugee status — the overall number was among the lowest since the turn of the century.

Nonetheless, Homeland Security Secretary John F. Kelly has somehow conjured what he called a “surge of illegal immigration at the southern border [that] has overwhelmed federal agencies and resources and created a significant national security vulnerability to the United States.” Mr. Kelly’s unfounded rhetoric is contained in a memorandum, released Tuesday, that provides an inventive rationale to justify the Trump administration’s overbroad expansion of deportation efforts. The effect of the new DHS guidelines is to sharply expand the pool of immigrants designated as priorities for deportation.

They do so by various means, including widening the targets of expedited deportation proceedings, until now limited to undocumented immigrants in the country for no more than two weeks and living within 100 miles of the border, to people who entered in the past two years and live anywhere in the nation — a cohort estimated at 800,000 to 1.1 million people. They also target not only people convicted of serious crimes but also those convicted of minor infractions, such as using a false Social Security number to get a job.

The guidelines’ subsidiary effects are just as concerning. They compromise law enforcement efforts in counties and cities nationwide by expanding efforts to deputize local police to act as federal deportation agents. That could chill cooperation between local law enforcement agencies and immigrant neighborhoods. The document sends a message of fear through many of America’s immigrant communities — not just the estimated 11.1 million unauthorized immigrants, but also their spouses, children and other relatives living legally in the United States.

Administration officials sought to ease such concerns by offering assurances that some enforcement measures will be implemented gradually, including an effort to return Central American refugees to Mexico as they await hearings on asylum claims. That will rely on coordination with Mexican authorities, whose cooperation may be dimmed by Mr. Trump’s hostility and his insistence that Mexico will pay for a wall it vehemently opposes.

Sean Spicer, the White House spokesman, likes to echo Mr. Trump’s comment that he has “a big heart,” the supposed evidence being that the DHS guidelines do not, for now, aim deportation efforts at ”dreamers” — the 750,000 young people given work permits and temporarily shielded from removal by the Obama administration. While that is welcome, in other respects a streak of cruelty runs through the new policy. For instance, it seeks to deter the entry of unaccompanied minors by threatening to prosecute parents if they paid smugglers to help their children cross the border. Deterrence is a fair goal if achieved by humane means. In this case, the administration’s policies will break up families and harm people leading peaceable lives.