The magistrate ruled that apprehended migrants may not have been aware they were crossing into a military zone.

A United States judge in the southwestern state of New Mexico has dismissed trespassing charges against dozens of migrants apprehended in a military zone recently created under President Donald Trump.

The military zone is one of two so far that the Trump administration has created along the US-Mexico border, in order to deter undocumented migration into the country.

Entering a military zone can result in heightened criminal penalties. As many as 400 cases have since been filed in Las Cruces, New Mexico, alleging security violations and crimes like trespassing on restricted military property.

But starting late on Wednesday and continuing into Thursday, Chief US Magistrate Judge Gregory Wormuth began issuing dismissals at the request of the federal public defender’s office in Las Cruces.

Wormuth ruled that the government had failed to demonstrate that the migrants knew they were entering a military zone.

“The criminal complaint fails to establish probable cause to believe the defendant knew he/she was entering” the military zone, Wormuth wrote in his orders dismissing charges.

The ruling is the latest legal setback for the Trump administration, as it seeks to impose stricter restrictions and penalties for undocumented immigration. But the president’s broad use of executive power has drawn the ire of civil liberties groups, who argue that Trump is trampling constitutional safeguards.

Establishing new military zones has been part of Trump’s strategy to reduce the flow of migration into the US.

Normally, the crime of “improper entry by an alien” carries fines or a prison sentence of up to six months. But trespassing on a military zone comes with steeper penalties than a typical border crossing, and Defence Secretary Pete Hegseth has warned of a possible combined sentence of up to 10 years.

“You can be detained. You will be detained,” Hegseth warned migrants. “You will be interdicted by US troops and border patrol working together.”

On April 18, the first military zone was unveiled, called the  “New Mexico National Defence Area”. It covered a stretch of about 274 kilometres — or 180 miles — along the border with Mexico, extending into land formerly held by the Department of the Interior.

Hegseth has said he would like to see more military zones set up along the border, and in early May, a second one was announced near El Paso, Texas. That strip was approximately 101km or 63 miles.

“Let me be clear: if you cross into the National Defense Area, you will be charged to the FULLEST extent of the law,” Hegseth wrote in a social media post.

Hegseth has previously stated that the military will continue to expand such zones until they have achieved “100 percent operational control” of the border.

Trump and his allies have frequently compared undocumented immigration to an “invasion”, and they have used that justification to invoke wartime laws like the Alien Enemies Act of 1798.

In a court brief on behalf of the Trump administration, US Attorney Ryan Ellison argued that the new military zones were a vital bulwark for national security. He also rejected the idea that innocent people might be caught in those areas.

“The New Mexico National Defense Area is a crucial installation necessary to strengthen the authority of servicemembers to help secure our borders and safeguard the country,” Ellison said.

He noted that the government had put up “restricted area” signs along the border. But the public defender’s office in New Mexico argued that the government had not done enough to make it sufficiently clear to migrants in the area that they were entering a military zone.

In the US, the public defenders noted that trespassing requires that the migrants were aware of the restriction and acted “in defiance of that regulation for some nefarious or bad purpose”.

Despite this week’s dismissals, the migrants involved still face less severe charges of crossing the border illegally.