President Trump’s long-awaited decision Monday to pare down and carve up two highly controversial national monuments in Utah has set off an unprecedented legal fight over the scope of an executive’s power to cede control of federal lands.

During a speech in Salt Lake City, Mr. Trump said he’ll reduce both the Bears Ears and Grand Staircase-Escalante national monuments, which were established by former Presidents Barack Obama and Bill Clinton, respectively.

The president cast his move as an effort to return control over land to local stakeholders, and to reverse a trend that saw administrations stretch the century-old Antiquities Act to its breaking point by using it as a tool to shut down huge swaths of land to energy development and recreation.

“Some people think the natural resources of Utah should be controlled by a small handful of very distant bureaucrats located in Washington. And guess what? They’re wrong,” Mr. Trump said. “The families of communities of Utah know and love this land the best and you know best how to take care of your land.”

While past presidents on 18 occasions have reduced monuments, none have done so on the scale Mr. Trumpannounced Monday. The Antiquities Act gives presidents the power to create monuments, but is silent on whether they have the authority to cancel or amend them.

Courts have never ruled on presidents’ power to shrink monuments, and opponents of Mr. Trump’s move immediately took their fight to court. Some legal scholars have said the battle, ultimately, could wind up before the Supreme Court.

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“Trump’s unprecedented, illegal action is a brutal blow to our public lands, an affront to Native Americans and a disgrace to the presidency,” said Randi Spivak, public lands program director at the Center for Biological Diversity. “He wants to hand over these lands to private industry to mine, frack, bulldoze and clear-cut until there’s nothing left for our children and grandchildren.”

The reductions have long been a policy goal of Sen. Orrin Hatch, Utah Republican, and other lawmakers who saw the Trump administration as a golden opportunity to finally reverse egregious federal land grabs. Supporters of Mr. Trump’s actions say they’re the beginning of true reform of how national monuments are created and managed.

“These new proclamations are a first step towards protecting identified antiquities without disenfranchising the local people who work and manage these areas,” said Rep. Rob Bishop, Utah Republican and chairman of the House Natural Resources Committee.

The Clinton administration’s 1996 creation of Grand Staircase-Escalante — done with virtually no consultation with Utah officials — was the first example of a president truly stretching the Antiquities Act.

The law states that a monument designation should be limited to the “smallest area” compatible with the artifacts or other historical items to be protected. Many other monuments across the country are relatively small and were created to protect specific items or locations of historical significance, such as the Stonewall Inn in New York City, an iconic location for the LGBT rights movement. But in the case of Bears Ears and Grand Staircase, hundreds of thousands of acres of wooded area were locked up, leading critics to charge that Democrats had found a legal loophole that allowed them to grab massive patches of land with impunity.