The Justice Department has issued a new opinion deciding that military personnel can arrest some illegal immigrants without running afoul of the Posse Comitatus Act.

That includes arresting illegal immigrants who trespass on military installations along the border, but it also means troops can pursue those migrants even after they leave Defense Department property.

Deputy Assistant Attorney General William D. Hyde Jr., who issued the finding Friday, said it doesn’t authorize “free license to enforce civil law.” But it does justify arrests at or near National Defense Areas, or NDAs, that the Trump administration has declared along the border.

Mr. Hyde said national security needs justify the arrest powers — even on lands that have been declared military property but don’t have any active operations.

“Even though substantial portions of the NDAs may be presently unoccupied or have no standing structures on them, there is a military necessity to ensure that unauthorized persons are not establishing a position to monitor the activities of U.S. forces for intelligence-gathering purposes or conducting reconnaissance in preparation for a terrorist attack,” Mr. Hyde concluded.

Part of the Trump administration’s successful effort to shut down the chaos at the southern border was to declare large swaths of land to be protected military property. The administration said troops had the power to police those areas, just as they could any military fort, base, camp or station.

NDAs have been established in all four southern border states, giving the military some authority over miles of the U.S.-Mexico boundary.

The new opinion comes from the Office of Legal Counsel, the Executive Branch’s in-house legal division that sets legal policy for the massive bureaucracy.

The Posse Comitatus Act generally prevents the military from conducting civilian law enforcement on U.S. soil.

There are some exceptions for drug interdiction activities.

Troops have also been roped into assisting Homeland Security with other border activities, including surveillance, migrant spotting and general mission support.

But the NDAs envision a more active role for troops to police the border itself.

The Justice Department began bringing charges for trespassing in NDAs last spring, soon after the first one was established near El Paso, Texas.

Thousands have been charged — though some skeptical judges have tossed cases, saying migrants didn’t know they were on Defense Department land so they lacked intent to commit the crime.

Mr. Hyde’s new opinion adds legal heft to the government’s argument for why the lands deserve protection.

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