The river runs jade until the Coast Guard comes.

Water ruffles up white against the boats as they gather speed.

A black vessel – open-air, with an armed crew of three – is new to the Rio Grande, where it patrols the international boundary near McAllen, Texas.

Petty Officer 1st Class Markus Graham grips the wheel in his helmet and shades.

The crew member calls out that he’s riding water where the United States and Mexico meet.

“We’re about dead center,” he says, tilting into a turn.

“We’re on the border.” The Coast Guard is used to occupying two realms at once.

Part of the military, it’s the sole branch that also sits in the Department of Homeland Security during peacetime, where its mission includes interdicting migrants and drugs.

Last fall, the service began sending people and boats to the southern border as part of a military buildup directed by President Donald Trump.

Yet the troops have arrived at a relatively quiet border.

Illegal crossings are down to thousands a month compared with the thousands per day during the Biden era.

Men, women, and children used to cross the Rio Grande en masse, linking arms in precarious human chains.

Now, Coast Guard members stationed here often scout empty riverbanks.

Deflated plastic rafts, from past attempts to cross, dot both shores.

Meanwhile, some federal border agents who typically patrol down here have headed north.

Those interior enforcement surges have been anything but quiet, as masked arrests, tear gas, protests, and the killings of two U.S.

citizens have roiled neighborhoods unaccustomed to chaos.

On the campaign trail, Mr.

Trump promised to close the southern border.

He counts that as an accomplishment now, even as more military muscle is sent to hold the line.

But the president has yet to check off another pledge: the largest deportation operation this country has ever seen.

That can only be accomplished in the interior, where millions of unauthorized immigrants have settled.

As the administration enters the second year of its whole-of-government immigration crackdown, the armed-forces escalation shows signs of shifting, but not slowing.

This dual dynamic – resources swelled to the border and interior at once – seems “unprecedented,” says Reece Jones, a geography professor at the University of Hawaii at Manoa, who studies borders.

While the government may claim control of the border up to a point, he says, claiming total control would invite more scrutiny of resource use.

“Because then the next question is going to be, When do we pull it back?” Soldiers, including National Guard members, have long aided border enforcement under Republican and Democratic presidents alike.

But over the past year, the Border Patrol has partnered with an expanded military presence numbering in the thousands – including the Coast Guard – as well as deputized members of the Texas National Guard.

The Pentagon has created military zones on public land.

These new “national defense areas” extend into the Rio Grande itself.

Meanwhile, U.S.

Customs and Border Protection (CBP) is keen to recruit, offering new Border Patrol agents bonuses of up to $60,000 over their first four years.

And on the same day that an immigration officer fatally shot Renee Good in Minneapolis in January, the Department of Homeland Security announced the arrival of over 500 miles of river-barrier buoys in Texas.

(The department will soon have a new leader: The president has fired DHS Secretary Kristi Noem amid mounting controversies tied to her leadership.) As reinforcements mount, the rural isolation along much of the border highlights the dichotomy of the unprecedented immigration enforcement effort.

In the interior, law enforcement has been met with protesters who the government says are impeding raids.

The borderlands lack the chorus of public witnesses, cellphones in hand, documenting federal actions in cities up north.

There are few places where the difference is clearer than in this part of south Texas.

In December, on the Rio Grande, the Monitor joined the Coast Guard and CBP for a look at the new normal.

Seen from a boat, stretches of border wall blur by twice: rising from the bank and mirrored in the water.

The river’s shallow depth can complicate navigation for Coast Guard arrivals.

That’s why their fleet includes a new Rock Boat, or Border Security Riverine, a vessel that Petty Officer Graham drives, which can skim over water mere inches deep.

The narrowness of the river – at times a few dozen yards wide – also limits response time when an illegal crossing is quick.

Towering carrizo reeds can conceal border crossers once they reach land.

For the crew members, it’s all an adjustment.

“If we’re not within a mile or two, it’s hard to catch anybody,” says Petty Officer 1st Class Christopher Bakri, wearing a beanie and driving a larger, 29-foot-long response boat.

While it can take people “maybe a minute” to cross the river, he says, “in the open ocean, you got all the time in the world.” On his bow and stern sit M240B machine guns, fed by cartridges that look like brass-colored fangs.

The plan is to “deter and disrupt illegal immigration, human traffickers, drug smugglers, and any other threats” that might try to cross from Mexico, says Capt.

Christopher Cumberland, commander of Coast Guard Forces Rio Grande.

“The adversary’s smart, well armed, well funded, well equipped.” By adversary, the commander says, he means Mexican cartels, criminal networks that the Trump administration now calls foreign terrorist organizations.

“We’ll support apprehensions, I would say, probably more than actually interdicting or detaining,” with handoffs to the Border Patrol for processing, he says.

Since the operation started in October, the Coast Guard reports supporting 179 apprehensions and deterring 181 other individuals who, after seeing the force, turned back to Mexico.

20, the Coast Guard also reports it has not seized any drugs.

(The DHS funding shutdown that began on Feb.

14 hasn’t halted the operation.) Captain Cumberland is among nearly 400 people staffing Operation River Wall, which the Coast Guard says has drawn from all its districts.

Even before then, a federal watchdog raised concerns about how such deployments could stretch the service’s capabilities.

In June, a Government Accountability Office report said that when the Coast Guard has prioritized assets for migrant....