United States troop deployments at the border with Mexico have tripled to 7,600 and include every branch of the military – even as the number of attempted illegal crossings plummets.

In addition, President Donald Trump has authorised funding for an additional 3,000 Border Patrol agents, offering $10,000 signing and retention bonuses.

The military mission at the border is guided from a new command centre at a remote Army intelligence training base located alongside southern Arizona’s Huachuca Mountains. There, a community hall has been transformed into a bustling war room, where battalion commanders and staff use digital maps to pinpoint military camps and movements along the nearly 3,200-kilometre (2,000-mile) border.

Until now, border enforcement had been the domain of civilian law enforcement, with the military only intermittently stepping in.

But in April, large swaths of the border were designated militarised zones, empowering US troops to apprehend immigrants and others accused of trespassing and authorising additional criminal charges that can mean prison time.

The two-star general leading the mission says troops are being untethered from maintenance and warehouse tasks to work closely with US Border Patrol agents in high-traffic areas for illegal crossings – and to deploy rapidly to remote, unguarded terrain.

“We don’t have a [labour] union. There’s no limit on how many hours we can work in a day, how many shifts we can man,” said Army Major-General Scott Naumann.

“I can put soldiers out whenever we need to in order to get after the problem, and we can put them out for days at a time. We can fly people into incredibly remote areas now that we see the cartels shifting [course].”

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The Trump administration is using the military broadly to boost its immigration operations, from guarding federal buildings in Los Angeles against protests, to assisting Immigration and Customs Enforcement (ICE) in Florida.

There are also plans to hold detained immigrants on military bases in New Jersey, Indiana and Texas.

Dan Maurer, a law professor at Ohio Northern University and a retired US Army judge advocate officer, said that Trump is aiming to follow through on his campaign promise to crack down on undocumented border crossings.

“It’s all part of the same strategy that is a very muscular, robust, intimidating, aggressive response to this – to show his base that he was serious about a campaign promise to fix immigration,” said Maurer.

“It’s both norm-breaking and unusual. It puts the military in a very awkward position.”