President Trump signed an executive order Wednesday on his central campaign promise that directs federal agencies to begin constructing a wall on the U.S.-Mexico border.

It was one of two executive orders on border security that Trump signed during a visit to the Department of Homeland Security (DHS).

“This is a law enforcement agency,” Trump said to applause, “But for too long, your offices and agents haven't been allowed to properly do their jobs.

“From here on out, I'm asking all of you to enforce the laws of the United States of America. They will be enforced and enforced strongly.

“A nation without borders is not a nation,” Trump said. “Beginning today, the United States of America gets back control of its borders.”

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Trump highlighted crimes committed by people in the country illegally, accentuating his point by introducing the family members of several victims and accusing the media of ignoring their stories.

“Pundits talk about how enforcing immigration laws can separate illegal immigrant families, but the families they don't talk about are the families of Americans," he said.

The second order signed by Trump would end the “catch and release” policy that quickly returned border-crossers to Mexico instead of arresting and processing them for deportation. The policy was a fixture of the George W. Bush administration and was later reinstated on an informal basis by former President Barack Obama Barack Hussein ObamaThe Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon Trump appointees stymie recommendations to boost minority voting: report Obama's first presidential memoir, 'A Promised Land,' set for November release MORE.

The immigration actions also seek to withhold visas from countries to make sure they take back people in the U.S. illegally who are found to have broken U.S. laws. It would also strip federal grants from “sanctuary” cities and states that do not enforce federal immigration laws.

Trump’s orders drew swift opposition from Democrats.

“I suspect that a lot of Trump supporters would be just as happy with a big statue of a middle finger pointed south, because both are about equally effective as national security strategies," Rep. Luis Gutiérrez (D-Ill.) said.

Hispanic leaders argued that Trump's plan to enlist local law enforcement officials to aid federal immigration authorities would only distance immigrant communities from local police.

Thomas Saenz, president of the Mexican American Legal Defense Fund said the Secure Communities Program — a joint local-federal immigration program revived by the executive order — "actually undermines security and communities, [and] will not make America great again."

Republicans hailed the moves as a welcome change after years of what they considered negligence on the part of Obama's DHS to enforce the nation's immigration laws.

“For far too long, the U.S. has been too lax with immigration policy at the expense of American citizens,” Rep. Michael Burgess Michael Clifton BurgessIgnore the misinformation: The FDA will ensure the safety of any COVID-19 vaccine House goes postal for one day GOP lawmaker calls asymptomatic testing crucial after CDC revises guidance MORE (R-Texas) said.

A number of questions remain about the president’s plans, including how to pay for a construction as massive as a border wall. Various estimates have put the cost at anywhere between $8 billion and $25 billion.

Trump has repeatedly said that Mexico will pay for the wall, but in an interview with ABC News on Wednesday, he acknowledged that U.S. funds would be needed to start the wall.

Trump said the federal government will “absolutely” be “reimbursed at a later date.”

“I'm just telling you there will be a payment,” Trump said. “As soon as we can. As soon as we can physically do it.”

Pressed on a timeline for when construction of the wall would begin, Trump said: “I would say in months. Certainly planning is starting immediately.”

Trump portrayed the wall as a benefit not only to American law enforcement officers working the border but also to the Mexican government “by deterring illegal immigration from Central America and by disrupting violent cartel networks.”

Mexico’s government seems unlikely to see things that way.

Mexican Secretary of the Economy Ildefonso Guajardo was attending a meeting at the White House as Trump signed the two orders. On Tuesday, he warned that a demand to pay for the wall would cause Mexico to withdraw from renegotiation talks on the North American Free Trade Act.

Mexican President Enrique Peña Nieto is slated to meet Trump in Washington next week, following up on meetings between top White House officials and his secretaries of foreign relations and the economy. But Peña Nieto has come under pressure from some politicians in his country to cancel the trip.

Existing legislation would authorize construction of border fencing, but Trump will likely need to ask Congress for money this year.

On the campaign trail, Trump initially suggested paying for the wall by taxing or impounding remittances — the money immigrants send to their families abroad. Some experts panned the idea, saying it would be legally impossible to single out Mexican remittances from those going to other countries.

The construction of the wall is expected to face major challenges, given the terrain it would have to cover and the areas it would have to pass through.

The U.S.-Mexico border goes through large metropolitan areas, such as El Paso-Ciudad Juarez and San Diego-Tijuana, which are largely walled off already; rural areas, including farms that in some cases straddle both sides of the border; and inhospitable wilderness, including national parks.

Of the almost 2,000 miles of border, 653 miles are already covered by a hodge-podge of fences mostly built since the late 1990s. Most of the fencing — 444 miles — was ordered built by the 2006 Secure Fence Act signed by Bush.

Wednesday's executive order mandates new construction under the same Secure Fence Act.

The Rio Grande forms a natural barrier along most of the eastern border region, leaving only the most remote and rugged areas exposed.

While the number of illegal border crossings has leveled off at around 400,000 a year since 2010 — comparable to levels in the 1970s — the number of Mexican nationals attempting to cross has continued to decrease.

In fiscal 2014 and 2016, Mexican nationals accounted for less than half of all illicit border crossings.

Jordan Fabian contributed.