Part of Bears Ears National Monument, near Blanding, Utah. (Rick Bowmer/AP)

PRESIDENT TRUMP entered the Oval Office an avowed enemy of environmental protection. This week the degree of his shortsightedness became clearer, with announcements suggesting he will aggressively undermine the measures his predecessors took to preserve precious lands and resources.

In Salt Lake City on Monday, Mr. Trump withdrew some 2 million acres of spectacular landscape from two national monuments in southern Utah: Bears Ears National Monument, which President Barack Obama declared late last year, and Grand Staircase-Escalante National Monument, which has enjoyed protection from drilling and other disturbances for two decades. This single move constituted the largest ever reduction in protected federal lands. Then on Tuesday it emerged that Ryan Zinke, Mr. Trump’s fox-in-henhouse interior secretary, will recommend paring back or loosening restrictions on 10 more national monuments around the country.

Several environmental and Native American groups immediately filed suit over Mr. Trump’s withdrawal, claiming that, while the president can unilaterally preserve land by creating or expanding national monuments, the White House cannot on its own withdraw acreage from existing monuments. The question has not been tested in court. But the groups face an uphill fight, considering that the law is extremely deferential to the president and stipulates that monuments should be only as large as is necessary to protect objects of interest requiring preservation. The administration will argue that Bill Clinton, George W. Bush and Mr. Obama each abused their authorities, setting aside vastly more federal property than necessary to preserve American treasures, such as the ruins scattered around Bears Ears and the geological wonders in Grand Staircase.

Yet while the legality of Mr. Trump’s move is debatable, its ill wisdom is less so. In general, public lands and waters should be open to development, including drilling, so that the nation’s resources can drive national growth, enable all sorts of recreation and benefit nearby towns and cities. But the federal government owns some lands that are simply too precious to permanently sully in pursuit of temporary economic gains. Bears Ears, with its spectacular canyons, buttes and unspoiled archaeological sites, is one such place. Grand Staircase, a natural wonderland of ancient topology and fossilized prehistory, is another. When administering such unique places, the government must err on the side of conservation.

Even as Utah ran an expensive national ad campaign encouraging Americans to visit the state’s natural treasures, its congressional delegation fought Bears Ears and they have cheered Mr. Trump’s announcement. Yet even for them, the reduction in size was radical. Utah officials had previously proposed protecting an area much larger than the truncated Bears Ears that Mr. Trump left as he departed Salt Lake City. Now it is their responsibility to fill the conservation gaps that the president has left.