President Donald Trump just signed an executive order that could cut the size of or abolish certain national monuments across the US, putting at risk historical sites and halting safeguards for public lands and waters. The move could eventually lead to litigation in the courts.

The executive order doesn’t resize or nix national monuments just yet. Instead, it directs the Department of the Interior to review the designation of tens of millions of acres of land set aside in the past 21 years, Interior Secretary Ryan Zinke told reporters at the White House last night. Zinke will then come up with recommendations for which monuments should be changed. At risk are some especially controversial places in Utah, like Bears Ears National Monument and Grand Staircase-Escalante National Monument.

At an event at the Interior Department today, Trump said the executive order would end an abuse of power that’s resulted in a “massive federal land grab.” “Today, I’m signing a new executive order to end another egregious abuse of federal power and to give that power back to the states and to the people where it belongs,” Trump said. “Today we’re putting the states back in charge. It’s a big thing.”

Through the years, US presidents of both parties have used the Antiquities Act of 1906 to set aside swaths of public land to protect important historical, cultural, and ecological sites without approval from Congress. The designation allows authorities to restrict activities such as grazing, farming, logging, and mineral mining. But protections vary from place to place. Generally, existing oil and gas development can continue, says Dan Hartinger, the national monuments campaign manager at the Wilderness Society.

President Barack Obama used the law to create new monuments or expand existing ones more than any other president. Some of the monuments he created were meant to commemorate African-American history, as well as gay rights. But some of the designations drew heavy criticism, including the Bears Ears National Monument, which protects 1.35 million acres of land in southeastern Utah. Many Republicans — including Sen. Orrin Hatch of Utah — vowed to fight the designation, which was seen an overstep of presidential authority.

The Antiquities Act doesn’t explicitly prohibit presidents from abolishing monuments, but a 1938 opinion by the US attorney general concluded that presidents have no such power, according to John Leshy, a former Interior solicitor who now teaches at the University of California Hastings College of Law. Some presidents have downsized monuments before: Woodrow Wilson shrank nearly by half Mount Olympus National Monument, which Theodore Roosevelt had created, according to The Washington Post. But no monuments have ever ever been abolished altogether, so there’s no precedent for this, says Kevin Book, the managing director of ClearView Energy Partners. The idea to just abolish an existing monument “is controversial at minimum and potentially subject to a legal challenge,” Book tells The Verge.

This is probably why Trump’s executive order for now just instructs the Interior Department to do a review and come up with recommendations. Zinke will first provide a recommendation on Bears Ears within 45 days; he’ll then come up with a final report within 120 days, according to the Associated Press. So Trump isn’t revoking national monuments just yet, “although it seems pretty obvious that that’s the ultimate goal,” Hartinger tells The Verge.

Zinke could make recommendations to Congress on which monuments should be resized or abolished. Though presidents don’t seem to have the power to rescind monuments, Congress very much does. “The Interior Department would be obliged to go through with the modification that Congress designates,” Book says. “The challenge, though, is doing it.” Even though Republicans control both houses, they’re far from united — as the fight over Obamacare showed.

Trump’s decision to review only monuments designated in the past 21 years suggests that he’s eying one particular monument, aside from the controversial Bears Ears — the Grand Staircase-Escalante National Monument, created by President Bill Clinton in 1996. The designation took many by surprise and sparked an outcry at the time, according to The Washington Post. “I guess they thought that opening it up to every single monument would seem completely ridiculous,” Hartinger says. “So they’re trying to find some middle ground, but that’s just a guess.”

Once the Department of the Interior finishes its review, that’s when litigation in the courts will likely begin. Environmental groups could sue the administration if it’s trying to shrink or eliminate national monuments. “The legal fireworks will start thereafter,” says Robin Craig, an environmental law professor at the University of Utah.

How those lawsuits will go isn’t clear, since there’s no precedent. But efforts to roll back protections of public lands and cultural sites might encounter fierce opposition from the public, Hartinger says. Polling shows that 90 percent of Americans support protecting public lands, parks, historical places, he says — and many of them are traditionally conservative constituents like hunters and fishermen. “Monuments are important to many different groups of people,” Hartinger says. “I think the public would be incensed if we were rolling protections for our history and culture and natural wonders.”