WASHINGTON (NYTIMES) - A federal appeals court in San Francisco ruled on Friday (June 26) that the Trump administration did not have the authority to transfer US$2.5 billion (S$3.5 billion) from the Pentagon to President Donald Trump's border wall without congressional approval, most likely sending the matter to the Supreme Court.

The 2-1 decision, which sided with environmental groups that brought the lawsuit, is the latest in what has become a legal odyssey centred on Mr Trump's choice to seize billions of dollars for the wall even after Congress expressly forbade it.

But the ruling will not immediately halt construction. Last July, the Supreme Court overturned a separate appellate decision and allowed the administration to move forward with wall building, using US$2.5 billion originally allocated to counterdrug programmes at the Defence Department.

Mr Dror Ladin, a staff lawyer at the American Civil Liberties Union's National Security Project, called the decision by the 9th US Circuit Court of Appeals "a win for the rule of law" that would help their argument before the Supreme Court.

"President Trump's xenophobic wall is already levelling protected lands, desecrating cultural sites and destroying wildlife," Mr Ladin said. "There's no undoing the damage that's been done, but we will be back before the Supreme Court to finally put a stop to this destructive wall."

Judges Sidney R. Thomas and Kim McLane Wardlaw, both appointees of President Bill Clinton, agreed with a lower court decision that the Trump administration did not have the authority to reallocate the funds without approval from Congress. They handed down their decision just days after Mr Trump travelled to Arizona to celebrate the construction of more than 320km of constructed wall.

"These funds were appropriated for other purposes, and the transfer amounted to 'drawing funds from the Treasury without authorisation by statute and thus violating the Appropriations Clause,'" Mr Thomas said in his opinion.

Judge Daniel P. Collins, an appointee of Mr Trump, dissented, saying the groups - the Sierra Club and the Southern Border Communities Coalition - lacked the justification to stop the transfer.

"To assist DHS by building fencing to stop international drug smuggling, the acting secretary of defence did not have to give even the slightest consideration to whether that reprogramming of funds would disrupt views of the desert landscape or affect local flora and fauna," Mr Collins wrote, referring to the Department of Homeland Security.

A department spokesman, Mr Alexei Woltornist, said on Friday that it was "sorely disappointed in this decision".

The administration is facing a separate legal challenge to the similar transfer of US$3.6 billion from Pentagon construction funds to build the border wall. The White House celebrated a win in that case in January when the 5th US Circuit Court of Appeals, in New Orleans, lifted a lower-court ruling to allow the administration to use the military funds.

Despite the court challenges, the administration has continued to tap the Defence Department for wall funds as Mr Trump tried to turn political attention to the border to help his struggling re-election campaign. The administration notified Congress this year that it would take another US$3.8 billion from the Defence Department for the wall, almost exactly a year after Trump used a national emergency declaration to initially circumvent Congress for his barriers.

"The funds that he is pilfering, which were appropriated by Congress, are vital to support the safety and well-being of the brave men and women in uniform, as well as their families," Speaker Nancy Pelosi of California said in a statement on Friday. "We will never allow our national security and the strength of our military families to suffer so that the president can fulfil an outrageous campaign promise."

With about US$15 billion of available wall funding, the administration has completed construction of 216 miles of new border wall. All but 3 of the miles of wall have replaced much shorter, dilapidated fencing or Normandy barriers.

"It's the most powerful and comprehensive border wall structure anywhere in the world," Mr Trump said this week even as Customs and Border Protection has requested ideas from the private sector on how to prevent people from climbing over the wall or cutting through it.

The White House is aggressively trying to meet a mandate set by Mr Trump to complete 724km by the end of the year, regardless of the tribal lands, archaeological sites, contracting laws and private owners that stand in the way.

The administration has waived many of those contracting laws, and this year it brought a flurry of lawsuits to acquire land from private property owners in South Texas, although the eminent domain process has proved to be arduous as some Texans aim to delay the construction beyond November.

Mr Ronald D. Vitiello, formerly Trump's acting director of Immigration and Customs Enforcement and chief of the Border Patrol, said the border wall could be effective in deterring illegal immigration if it is complemented with technology and patrolling agents.

"In itself, it doesn't do much, but it establishes an anchor, a base, for all the required attributes," Mr Vitiello said. "You got to have technology to support it and agents to make arrests when people still attempt to climb it."

Mr Peter Vincent, the former highest-ranking lawyer for ICE, said the construction of the border wall had actually harmed public safety since Mr Trump had transferred billions of dollars from the Defence Department to secure the funding.

"I've always said, based on my experience in counterterrorism and anti-narco trafficking, a wall does not in any way prevent a committed nefarious actor from breaching our southern border by either climbing over it, going around it and digging underneath it," said Mr Vincent, who also served as ICE's senior counsellor for international policy.

"It's pretty obvious President Trump is looking for any distraction he can gin up to move public attention away from his horrific handling of the coronavirus pandemic," he said. "He's gone back to his tried, trusted and true - yet completely incorrect - view of border security, including touting his border wall."