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On February 15, 2019, Donald Trump declared a National Emergency Concerning the Southern Border of the United States. However as far as I can tell, construction of the actual wall has yet to begin despite the national emergency. What is the reason for this?

The tl;dr is that a federal injunction is preventing the building of the wall, because it's likely that the Executive branch exceeded its Constitutional authority in trying to use the NEA (National Emergencies Act) to bypass Congress.

This is how our system of checks and balances works.

For those less familiar with the US system, it was designed to have three co-equal branches of government who are supposed to keep each other in check. In short:

• 
  Legislative: Congress, in charge of creating laws


• 
  Executive: The President & basically all other federal agencies, in charge of enforcing laws


• 
  Judicial: The Supreme Court & all lesser federal courts, in charge of interpreting and applying the laws, as well as ensuring they conform with the Constitution

In this case, deciding where money is spent, is a power given to the Legislative branch. The Executive was given the power to act quickly when Congress is unable to decide under the NEA by Congress. But, because Congress already had a chance to act and did not, the President lost his powers under the NEA.

Given this situation, the proper recourse for the President would be to go back to Congress and petition them for the funding. This would probably require some sort of deal, given his party is not in control of both chambers of Congress. Certainly he could attempt some other method of circumventing Congress, but the separation of powers is pretty clear in this respect, so it's unlikely to succeed.

The reasoning, however, I think is worth expanding on:

You can read the ruling in full here. All the following quotes are from this source.

President Trump used the National Emergencies Act of 1976, which was enacted

“to insure that the exercise of national emergency authority is responsible, appropriate, and timely.” Comm. on Gov’t Operations & the Special Comm. on Nat’l Emergencies & Delegated Emergency Powers, 94th Cong., 2d Sess., The National Emergencies Act (Public Law 94–412)

(page 13)

Furthermore, they contemplated and dismissed Presidential veto during deliberations:

Mr. Conyers. . . . Mr. Chairman, my final participation in this debate
revolves around the reason of this question: What happens if the
President of the United States vetoes the congressional termination of
the emergency power? Is that contemplatable within the purview of
this legislation?

. . .

Mr. Flowers. Mr. Chairman, on the advice of counsel we have
researched that thoroughly. A concurrent resolution would not
require Presidential signature of acceptance. It would be an
impossibility that it would be vetoed.

Mr. Conyers. So there would be no way that the President could
interfere with the Congress?

Mr. Flowers. The gentleman is correct.

(page 14)

This was subsequently overruled by the Supreme Court and the law amended.

Also important is that in this case: Congress said no and the President is attempting to circumvent them.

This appears to have been the first time in American history that a
President declared a national emergency to secure funding previously withheld by Congress.

(page 8)

To which the judge rules:

The Court agrees with Plaintiffs that they are likely to show that the proposed transfer is for an item for which Congress denied funding, and that it thus runs afoul of the plain language of Section 8005 and 10 U.S.C. § 2214(b) (“Section 2214”).

In fact, this is the first of three major points towards the injunction on the legality of the reprogramming of funding:

Plaintiffs are Likely to Show That the Item for Which
Funds Are Requested Has Been Denied by Congress.

Plaintiffs are Likely to Show That the Transfer is Not
Based on “Unforeseen Military Requirements.”

Accepting Defendants’ Proposed Interpretation of
Section 8005’s Requirements Would Likely Raise Serious
Constitutional Questions.

They conclude with:

The Appropriations Clause is “a bulwark of the Constitution’s separation of powers among the three branches of the National Government,” and is “particularly important as a restraint on Executive Branch officers.” Id. at 1347. In short, the position that when Congress declines the Executive’s request to appropriate funds, the Executive nonetheless may simply find a way to spend those funds “without Congress” does not square with fundamental separation of powers principles dating back to the earliest days of our Republic.

(pages 54-55)

It's a very large civil engineering project. Those are nearly always late. It's supposed to be at least six times longer than the Berlin Wall, and the cost projections are in the $20bn range.

There is a GAO report on the subject, which if we skip to the conclusion:

DHS plans to spend billions of dollars developing and deploying new barriers along the southwest border. However, by proceeding without key information on cost, acquisition baselines, and the contributions of previous barrier and technology deployments, DHS faces an increased risk that the Border Wall System Program will cost more than projected, take longer than planned, or not fully perform as expected. Without assessing costs when prioritizing locations for future barriers, CBP does not have complete information to determine whether it is using its limited resources in the most cost-effective manner and does not have important cost information that would help it develop future budget requests.

This suggests to me that CBP (Customs and Border Protection) is not very good at handling large civil engineering projects, and are very likely to experience cost overruns. They appear to still be at the phase of evaluating wall design for effectiveness.

(The start of the PDF is interesting in giving the history, which goes back to 1996, of trying to construct more barriers at an ever-widening set of "critical points".)

Politically, the declaration of an "emergency" means little to nothing. This is not a great national motivator like the moon landings, it's a highly controversial and probably ineffective scheme being carried out for partisan purposes. The leadership is not interested in the kind of detailed followup and programme management that would accelerate it. So it's likely to overrun endlessly.

The main effect of the emergency declaration was to allow Trump to divert funds from other budget categories to this project. However, there have been court challenges, and according to the Wikipedia page you linked to:

On May 24, 2019, in response to lawsuits brought by the Sierra Club and the Southern Border Communities Coalition, U.S. District Judge Haywood S. Gilliam, Jr. temporarily blocked the Trump administration's plan to divert funds not explicitly appropriated by Congress. Gilliam, an Obama appointee, wrote that "Congress's 'absolute' control over federal expenditures—even when that control may frustrate the desires of the Executive Branch regarding initiatives it views as important—is not a bug in our constitutional system. It is a feature of that system, and an essential one."

There was also a lawsuit from the House of Representatives, but the judge in that case ruled that the House doesn't have standing to sue the President.

The first lawsuit will likely be appealed, and most expect it to get to the Supreme Court. But until that decision, the emergency diversion is blocked.