Before President Donald Trump took office, fencing and other barriers stood along 654 miles of the 1,900-mile border with Mexico. The administration has replaced or upgraded a tenth of that existing network — without building much in the way of new walls. Now Trump is determined to deliver on his signature 2016 campaign promise in time for the 2020 elections. But even as construction begins on parts of the wall, it is unlikely to fundamentally alter illegal border crossings of people or drugs.

Trump has scaled back his initial promise to build a wall along the entire length of the frontier, saying that 1,000 miles of new barriers should suffice. He is pressing to complete nearly half of that by the end of next year, having deputized his son-in-law, Jared Kushner, to ride herd on the project.

That's a tall order given that the government will have to fight court battles with private landowners to take much of the land where the wall would be built. Other impediments include objections arising from the wall's environmental impact; opposition by congressional Democrats, who are in no mood to appropriate the $5 billion the White House is demanding for the wall in the current fiscal year; and, most recently, a federal court order blocking the administration's plan to divert $3.6 billion for wall construction from funds approved by Congress for projects on military bases at home and overseas.

Those fights may intensify as the wall's overall cost, which Trump once put at $8 billion, turns out to be several times that amount. In the meantime, it's worth bearing in mind what the wall might and might not achieve.

It might provide an additional deterrent to unauthorized crossing for some migrants not already deterred by policy obstacles the administration has adopted. However, the border wall will not be an impregnable barrier. As The Washington Post's Nick Miroff wrote last month, Mexican gang members have already figured out that a cordless household tool, available at hardware stores for as little as $100, can slice through the steel-and-concrete bollards from which the new wall will be built.

Nor is it likely to drastically reduce overall numbers of unauthorized migrants in the United States, who for most of the past decade have included many more people who overstay their visas than who cross the border illegally. A report this year by the Center for Migration Studies calculated that about two-thirds of new illegal presences in recent years have been overstays, who now account for more than 40% of some 11 million unauthorized immigrants.

The wall is also unlikely to address contraband smuggling, including of heroin and fentanyl, a deadly synthetic opioid. Most of those substances enter the country through legal ports of entry along the southwestern border, hidden in cars and trucks.

The president has convinced many of his supporters that the wall, which Mexico is not paying for, is worth any expense or funding method. The rest of the American public is justified in its skepticism.

This editorial is the opinion of The Washington Post's editorial board.