For anyone still confused as to why Trump will at least win a portion of the border wall battle...

You should read this article from the blog "Lawfare". It's a fairly straightforward objective discussions of the facts. It doesn't pretend either way to filter this with any politics. It's neither pro-Trump or anti-Trump, and it avoids political rhetoric.

In other words, it's just a plain and simple explanation of the law.

For those who are not fully informed, the President did not just declare a national emergency, but he actually wrote three executive orders. While it seems clear that there will be litigation regarding the so called National emergency (which is $3.6 billion of the additional monies Trump is moving towards the wall), there is almost no basis for anyone to contest the other two orders

From the article:

First, according to the fact sheet, $601 million would come from the Department of the Treasury Forfeiture Fund, an account that is fed by money seized by the U.S. government in connection with law enforcement actions or generated by the sale of seized assets. No national emergency declaration is needed to tap these funds. 31 U.S.C. § 9705(g)(4)(B) authorizes the Forfeiture Fund to use the funds for specific purposes outlined in that section:

After reserving any amount authorized by paragraph (3)(C) and after transferring any amount authorized by paragraph (3)(A), any unobligated balances remaining in the Fund on September 30, 1994, and on September 30 of each fiscal year thereafter, shall be available to the Secretary, without fiscal year limitation, for transfers pursuant to subparagraph (A)(ii) and for obligation or expenditure in connection with the law enforcement activities of any Federal agency or of a Department of the Treasury law enforcement organization. (Emphasis added.)

In other words, after reserving funds to provide for the next year’s fiscal operations, remaining funds (“unobligated balances”) are available in connection with the law enforcement activities of any federal agency—including the Department of Homeland Security or any other agency that may be involved with building the wall. While Congress must periodically reauthorize the fund, the money in the fund does not “expire” like normal appropriated funds do. Rather, the funds are what’s called a “permanent indefinite appropriation” provided by Congress.

Second, the fact sheet indicates that $2.5 billion would come from funds authorized pursuant to 10 U.S.C. § 284, which likewise does not depend on a declaration of emergency. This statute authorizes the secretary of defense to “provide support for the counterdrug activities or activities to counter transnational organized crime of any other department or agency of the Federal Government or of any State, local, tribal, or foreign law enforcement agency” for the purposes set forth in the section if requested by another federal agency or state, local or tribal government in support of law enforcement efforts. Those purposes include, among other things, “[c]onstruction of roads and fences and installation of lighting to block drug smuggling corridors across international boundaries of the United States” (emphasis added). Presumably the administration has decided the wall, or at least parts of it, is really a “fence”—and as such that these funds will be available for the project.

Regarding the amount of funds available pursuant to 10 U.S.C. § 284, the fiscal 2019 Department of Defense appropriations bill provides for only $881,525,000. It is not clear where the balance of the $2.5 billion would come from. Funds would presumably need to be transferred or reprogrammed from other sources pursuant to other authorities. Congressional staff were told on Feb. 15 that the administration is currently working on exactly how that would be done.

So ultimately, the President should be able to add an additional $1.5 billion dollars to the $1.4 provided by Congress by doing nothing other than following the law.  Not only would it be perfectly legal to do so, but it seems almost prudent that the President would use money set aside for drug enforcement on building a southern border barrier.

Where arguments (and legal actions) might occur is in regards to where the additional 2.5 billion might come from (regarding 10 U.S.C. § 284) and of course the actual declaration of a national emergency.

In terms of the national emergency, this article suggests:

Third, $3.6 billion would be drawn from the Department of Defense military construction budget. 10 U.S.C. § 2808(a) provides:

“[i]n the event of a declaration of war or the declaration by the President of a national emergency in accordance with the National Emergencies Act (50 U.S.C. 1601 et seq.) that requires use of the armed forces, the Secretary of Defense, without regard to any other provision of law, may undertake military construction projects, and may authorize the Secretaries of the military departments to undertake military construction projects, not otherwise authorized by law that are necessary to support such use of the armed forces. Such projects may be undertaken only within the total amount of funds that have been appropriated for military construction, including funds appropriated for family housing, that have not been obligated.” (Emphasis added.)

This is the only portion of the funds that requires the declaration of a national emergency. One of us (Taylor) evaluated this authority in detail on Lawfare last month.

Congress estimates the Pentagon has $21 billion in already-appropriated funds for military constructions projects as of November 2018, appropriated since fiscal 2015. While a large portion of these projects are awarded and in progress, the administration would presumably seek to avoid disrupting ongoing projects—though they could delay some projects to make money available for the wall. The Department of Defense has indicated to congressional staff that it does not intend to include family housing projects in the evaluation of projects that could be delayed.

In other works, Congress authorizes money for the Pentagon for military constructions, but ultimately it's up to the Pentagon to decide where that money is spent. The President (as ultimate boss of all executive branch agencies) would have constitutional authority to make certain determinations as how the 21 billion will be spent, at least under the pretense of a national emergency.

This, of course, is quite different from the rhetoric that Congress has some sort of authority/responsibility/duty to micromanage all appropriate funding. But it's sort of refreshing to read something "factual" that relies on the statutes and laws, rather then read partisanship, rhetoric, and political misdirection that we are used to.

There is some additional information as to where they believe these orders are most vulnerable (not surprisingly they see the most potential problems with the appropriations of funds coming from 10 U.S.C. § 2808). Although like most other objective legal scholars they do not believe that challenging the concept of a national emergency has much chance of success. They see more issues with whether or not a border barrier can be considered a military operation, and expect litigation to follow down that path.

They seem to suggest (without making any specific predictions) that when the dust settles that it will be a mixed bag of legal results, but that ultimately the goal from the opposition will be about delaying the building of the wall, rather than actually preventing it.

Time will tell.