Immigration crackdown advocates and some legal scholars had urged the Trump administration to revoke the memo when it came into office in 2017, but that didn’t happen.

The memo would come back to bite the administration as it later tried to phase out DACA, arguing the program was illegal, even though the Justice Department’s own guidance said otherwise.

Mr. Barr revoked the OLC memo in June, though the move wasn’t revealed publicly until Tuesday, when Homeland Security released a letter Mr. Barr wrote to the department concerning DACA. In it, Mr. Barr said he was also erasing guidance former Attorney General Jeff Sessions had given on DACA’s legality.

“I wish to wipe the slate clean,” Mr. Barr wrote to acting Homeland Security Secretary Chad Wolf.

Mr. Wolf announced an update of the DACA program on Tuesday, cutting the period of amnesty from two years to one year for those renewing their status, and blocking any new applicants from joining the program.

He also announced a full legal review with an eye toward determining whether to phase out DACA altogether.

President Trump’s first attempt at a phaseout was erased by the Supreme Court in a 5-4 decision last month — though the justices said he could try again, if he gave better reasons.

President Obama created DACA in 2012. He’d spent years saying he didn’t have power to grant a blanket deportation amnesty to illegal immigrants, but reversed himself in the months before the election and decided he had such powers after all.

Officially known as Deferred Action for Childhood Arrivals, the program protected Dreamers, young adult illegal immigrants who came to the U.S. before age 16, had pursued education and kept a relatively clean criminal rap sheet. More than 800,000 people have been approved over the eight years of the program.

In 2014, Mr. Obama attempted to expand the policy, announcing Deferred Action for Parental Accountability, which could have granted similar deportation protections and work permits to as many as 4 million other illegal immigrants who had U.S. citizen or legal permanent resident children.

The OLC memo said since those children could eventually sponsor the parents for legal status years in the future, it was legal for the president to block their deportation in the meantime.

The memo also gave a belated justification to DACA.

It said DACA was legal as long as it wasn’t an automatic grant of authority, and Homeland Security officers could decide each case individually.

In practice, more than 99% of DACA applications were approved in the early months of the program, and even in the last days of the Obama administration nearly 85% were approved.