President Trump’s proposed wall along the U.S.-Mexico border cannot be built with only the executive order he signed Wednesday and its construction will require congressional approval, border experts and former federal officials said.

While Trump can start the wall by shifting around existing federal funds, he will need Congress to appropriate the $20 billion — and perhaps significantly more — required to complete the massive structure, the experts and former officials said.

“How is he going to fund it? You need money!” Rand Beers, a former acting Department of Homeland Security secretary in the Obama administration, said Wednesday. “He’s got to have the money. And you can’t reprogram all that money without congressional authorization.”

Trump’s order, signed during a visit Wednesday to the Department of Homeland Security, mandates that DHS Secretary John F. Kelly “immediately plan, design, and construct a physical wall along the southern border.’’

It is vague about funding, ordering Kelly to “identify and, to the extent permitted by law, allocate all sources of Federal funds’’ for the project. But the order appears to acknowledge that congressional approval will be needed to finish the structure, saying Kelly also should “develop long-term funding requirements for the wall, including preparing Congressional budget requests for the current and upcoming fiscal years.’’

(Zoeann Murphy/The Washington Post)

[Mexicans want their president to cancel visit to Washington to see Trump]

While congressional Republicans have said they plan to fund the wall, administration officials have said they are still discussing specific funding options with GOP lawmakers. Trump vowed Wednesday that construction will begin within months, and White House officials said this week that the former developer is likely to personally oversee aspects of its construction.

The action on the proposed wall was one of two presidential decrees Trump signed Wednesday to begin what administration officials are privately calling “immigration week.” Trump ordered a broad crackdown on the nation’s estimated 11 million illegal immigrants, mandating more detention centers, additional federal border control agents and a withholding of federal funds to cities that do not comply with federal immigration laws.

The wall was Trump’s signature and perhaps most controversial proposal in a presidential campaign that centered on his planned crackdown on illegal immigration. In an interview in 2015 with The Washington Post, the then-presidential candidate said the structure will be “easy” to build and “can be done inexpensively.”

But border security experts and former DHS and congressional officials said Wednesday that the project is a massive and difficult undertaking. In addition to the cost, they said, it would face engineering and environmental problems; fights with ranchers and others who don’t want to give up their land; and the huge topographical problems of the border, which runs through remote desert in Arizona and rugged mountains in New Mexico, as well as, for two-thirds of its length, along rivers.

“Building the wall would be an unprecedented undertaking, logistically, financially and resource-wise,” said Joanne Lin, senior legislative counsel at the American Civil Liberties Union. “And it’s not going to be erected overnight either. It’s a huge expense.’’

The Trump administration will have to contend with private ranchers, farmers and other property owners with land along the border. Just one-third of the border is made up of federal and tribal lands, according to a 2015 report by the Government Accountability Office, with private and state-owned lands making up the rest.

The relationship between the U.S. Border Patrol and these private landowners has often been rocky, as ranchers complain that illegal immigrants damage their property as they travel north from the border — or that border patrol agents cause problems as they apprehend fleeing migrants.

In recent years, the Border Patrol has boosted training for new agents to improve relations with local landowners. Agents all carry keys on their belts that they must use to open gates of private lands when they are patrolling or apprehending, their access granted after individual negotiations with property owners.

There are more than 45 walls and border fences worldwide, perhaps most prominently Israel’s West Bank barrier. In the United States, the concept of a wall or fence along virtually the entire border has bubbled up occasionally in the nation’s immigration debate, with some Republicans supporting the idea.

The U.S. government began building border fencing near San Diego in 1990. After Sept. 11, 2001, amid a broader crackdown on illegal immigration, President George W. Bush dramatically expanded the effort.

Overall, more than $7 billion has been spent to build what is now about 650 miles of Southwest border fencing — costing nearly $5 million per mile in some spots — nearly half in Arizona.

But while current and former DHS officials say the fencing has been effective in deterring illegal immigration, it has also been beset by problems. The fencing mandated by Congress in 2006 faced delays, surging construction costs and disputes with private property owners, mostly in Texas.

Experts said Wednesday that the Trump administration is likely to be able to use eminent domain to get the land needed to build the wall, citing a 2005 Supreme Court decision that said local governments may force property owners to sell and make way for private economic development when officials decide it would benefit the public.

But experts said the process could involve years of legal challenges.

Karoun Demirjian contributed to this report.

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