Updated Wednesday at 10:40 a.m. with comments from Ken Paxton, John Cornyn and Henry Cuellar.

WASHINGTON -- Frustrated by Congress' refusal to provide funds for more than a modest length of border wall, President Donald Trump declared Tuesday that he will deploy U.S. troops to the frontier with Mexico.

He also dramatically scaled back his demand for a barrier along the border, saying he wants a wall along just "700 to 800 miles" -- a far cry from what he led supporters to expect.

The White House and Pentagon did not provide details of the military deployment Trump envisions, leaving key questions unanswered: how many troops, how heavily armed they would be, their mission, or how success will be defined.

"Until we can have a wall and proper security, we're going to be guarding our border with the military. That's a big step. We really haven't done that before -- certainly not very much before," Trump said during a White House visit with the leaders of Latvia, Lithuania and Estonia. "We cannot have people flowing into our country illegally, disappearing, and by the way never showing up for court."

Militarizing the border would likely escalate tensions with Mexico, which is in the midst of a presidential campaign in which a leading candidate has vowed to ensure that his country is no longer a "piñata" for foreign powers. Trump has been threatening to scrap NAFTA -- both to secure more favorable trade terms, and to pressure Mexico to curb the flow of its own citizens and migrants from Central and South America.

But Trump's inability to deliver on his signature campaign promise has left backers deflated and the president himself openly flustered. He briefly threatened to veto the $1.3 trillion budget deal over the shortage of wall funds.

Duncan Wood, director of the Wilson Center's Mexico Institute, questioned the justification for a military deployment and called it "an extraordinary escalation."

Gov. Greg Abbott has already deployed Texas National Guard to the border, and state troopers, as he noted late Sunday via Twitter.

Texas already has the National Guard and Dept. of Public Safety deployed to the border. https://t.co/HFNfC9ckRH — Greg Abbott (@GregAbbott_TX) April 2, 2018

Since the 1980s, federal military units and National Guard have provided support to civilian agencies along the border, working on counterdrug and counterterrorism missions.

Rep. Beto O'Rourke, the El Paso Democrat hoping to oust Sen. Ted Cruz, called Trump's proposal dangerous to U.S. citizens and immigrants alike. He recalled the 1997 incident in which a Marine on a drug patrol shot and killed Ezequiel Hernandez, an 18-year-old American who was tending goats near the border at Redford, Texas, south of Presidio.

This is what happens when we militarize the border. 18-year old Ezequiel Hernandez, U.S. citizen, killed by a U.S. service member patrolling the border. https://t.co/0B3cjzlAEN — Beto O'Rourke (@BetoORourke) April 3, 2018

Texas Attorney General told Fox News on Tuesday night that "If the president thinks the National Guard will help us, given the situation in Mexico and this caravan of people, certainly we're open to that, if it provides greater security and protects our citizens."

On Wednesday, Sen. John Cornyn of Texas, the deputy Senate majority leader, offered Trump some political cover, noting on Twitter that "deploying troops to the border is not unprecedented."

Deploying troops to the border is not unprecedented. The Obama administration sent 1,200 National Guard troops to the southern border in 2010 to assist Border Patrol and immigration officials amid rising concerns about drug trafficking. @TexasTribune — Senator John Cornyn (@JohnCornyn) April 4, 2018

But borderland congressman Henry Cuellar, a Laredo Democrat, rejected Trump's approach. "Deploying troops to secure a border and constructing a 14th Century concrete wall, are outdated and inadequate answers to protecting our nation's borders," he said in a statement.

Trump vowed throughout the 2016 campaign to build a "big, beautiful wall" along the 1,954-mile U.S.-Mexico border, and his rhetoric was widely regarded as a call for a full-length barrier.

Modest expansion

About 654 miles of the border is already fenced, so Trump's new goal represents only a modest expansion. Congress provided $1.6 billion in the recent $1.3 trillion omnibus budget, enough to add 33 miles and for replacement barrier along twice that length.

"We have $1.6 billion, and we're starting brand-new sections of walls. But we need to have a wall that's about 800 miles -- 700 to 800 miles of the 2,000-mile stretch. We have a lot of natural boundaries," Trump said at a White House meeting, with Defense Secretary Jim Mattis at his side.

Trump has generally been vague about many miles of barrier he envisions. Last July, he told reporters aboard Air Force One as he flew to Paris that he wants "anywhere from 700 to 900 miles of see-through wall." The remarks were initially off the record but the White House later agreed to release them.

Last Friday, his Border Patrol chief said the administration's goal is 1,000 miles, including both new and replacement barrier. And even that suggested a major retreat from campaign promises.

Previous presidents have sent federal troops to shore up security and reinforce the Border Patrol.

In 2006, President George W. Bush ordered 6,000 troops to the border in support roles such as aviation and logistics, to free up Border Patrol agents to focus on catching and detaining immigrants in the country illegally. Operation Jump Start lasted two years, ending in July 2008. President Barack Obama deployed 1,200 troops in 2010, under pressure from Rick Perry -- then Texas' governor and now Trump's energy secretary -- to send at least 3,000.

Both presidents offered assurances that they were not trying to "militarize" the border -- something Trump has not said so far.

"We are preparing for the military to secure our border between Mexico and the United States," Trump reiterated later Tuesday at an East Room news conference. "It's something that we have to do."

WE WILL PROTECT OUR SOUTHERN BORDER! pic.twitter.com/Z7fqQKcnez — Donald J. Trump (@realDonaldTrump) April 3, 2018

A month after Trump took office, news reports indicated that he was considering mobilizing 100,000 National Guard for border missions.

The White House vehemently denied those reports.

More reaction

State Sen. José Rodríguez, an El Paso Democrat whose district covers about 350 miles of the border, accused Trump on Tuesday of manipulating public fears, using troops "to create electoral theater in hopes of appeasing his political base."

"We are not Russia or any other totalitarian country that uses our military domestically, against our own residents," he said, adding, "Border communities do not want the military patrolling their backyards."

"The Texas border is a home, not a war zone," said Texas Democratic Party Chairman Gilberto Hinojosa. "Shame on him for using our military as a political prop for his stupid wall."

But Rep. Ted Poe, R-Humble, lauded Trump's plan, tweeting that "protecting American sovereignty and the American people" is a federal responsibility.

Wood, the Wilson Center Mexico expert, noted that Trump just pushed Congress to beef up the Pentagon budget in the $1.3 trillion spending deal, "ostensibly to build America's strength in the world."

Shifting resources "to a border that is far from threatened," he said by email, is "extremely incongruous."

Apprehensions low

Apprehensions at the border, a key measure of illegal flows, are at a nearly 50-year low. Trump himself has boasted that migrant flows have dropped. Seizures of cocaine, marijuana and heroin haven't been this low in years.

But the president has voiced frustration at immigration laws he deems lax and a lack of authority to quickly deport people caught entering the country illegally. He has expressed alarm at an annual "caravan" of 1,500 Central American migrants walking through Mexico to highlight the plight of displaced workers; as of Thursday the group was 800 miles from the U.S. border

The big Caravan of People from Honduras, now coming across Mexico and heading to our “Weak Laws” Border, had better be stopped before it gets there. Cash cow NAFTA is in play, as is foreign aid to Honduras and the countries that allow this to happen. Congress MUST ACT NOW! — Donald J. Trump (@realDonaldTrump) April 3, 2018

"Mexico is already cooperating at historic levels," Wood said. "...Is this a gambit to get Mexico to do more in stopping the flows or to give more concessions on NAFTA? Or is this a domestic political calculation designed to pressure Congress and appeal to hard line voters who see immigration as a threat? Either way this creates horrible optics for the US."

The federal Posse Comitatus Act precludes the use of military for law enforcement. National Guard operating under state authority are exempt. But that would require Trump to enlist the cooperation of border state governors -- and for states to bear the cost.

Defense Secretary Jim Mattis looks on as President Donald Trump speaks during a working lunch with Baltic leaders, at the White House on April 3, 2018. Trump, who has been stewing publicly for days about what he characterizes as lax immigration laws, said here that he plans to order the military to guard the border with Mexico. (DOUG MILLS / NYT)

Critics blasted Trump's proposal, even without details.

"Trump's border obsession is the mark of a weak, wobbly presidency," Randy Serraglio, conservation advocate at the Center for Biological Diversity, a group that has sued to block wall construction, said by email. "Militarizing the border is a disaster for communities, wildlife and the borderlands and does nothing to address immigration issues."

The troop deployment idea comes after a series of setbacks for Trump. Seven months after he scrapped the Deferred Action for Childhood Arrivals program, the Obama-era policy that shielded younger immigrants from deportation, Congress has not renewed it.

Lawmakers rejected Trump's efforts to secure $25 billion for wall construction in exchange for a DACA fix.

Throughout the campaign Trump left an impression that he wanted to wall off the entire border.

A year ago, his first homeland security secretary, John Kelly -- now White House chief of staff -- assured Congress that it made no sense to build a "sea to shining sea" wall along rivers, canyons and remote rugged mountains.