Build that wall! Eventually. Or at least patch up some existing fencing. If Chuck and Nancy will agree.

Donald Trump’s signature pledge to build a border wall, aka The Wall, is diminishing almost by the hour. No battle plan survives contact with the enemy, said Helmuth von Moltke the Elder, the 19th-century German field marshal. He left no record of his view of what happens when an emotive slogan dressed up as a policy proposal comes into contact with the enemy, but we can assume that he didn’t think it would fare well.

It hasn’t. Trump is paying the price for making The Wall the most powerful symbol of immigration restriction, when it isn’t particularly important or achievable. He piled lurid fantasy on top of absurd overpromising by asserting that Mexico would somehow be made to pay for the barrier.

This worked brilliantly for Trump during the campaign. His rally-goers delighted in his familiar lines about The Wall and engaged in call-and-response with the candidate like the chorus at an old-time revival. The Wall underlined Trump’s tough image and larger-than-life persona. Just as Rome had its Aurelian Walls, America would have its Trumpian Wall -- some 1,000 miles long, impenetrable and altogether “big and beautiful.”

Then the bill came due. Or, to be more precise, it didn’t. There was no way, absent the threat of a punitive U.S. invasion, that the government of Mexico was going to suffer the national humiliation of paying for a Yanqui border wall. In the leaked transcript of a call with Mexican President Enrique Pena Nieto, Trump pleaded with Pena Nieto just to stop saying that Mexico wouldn’t pay for The Wall. And couldn’t even manage that.

So the U.S. government would have to fund The Wall after all (although only as a down payment until Mexico paid its arrears, according to Trump). This was a problem because no one truly believes that a vast border wall -- traversing remote territory, causing immense legal complications over the right of way, and costing billions of dollars -- makes much sense. Yes, key areas need more robust fencing, but that doesn’t require replicating the Walls of Constantinople.

The Trump administration climbdown began immediately. In his confirmation hearings for Department of Homeland Security secretary, John Kelly referred to “high-tech fencing,” i.e., sensors and the like. Trump himself has fluctuated between insisting that The Wall will be built -- at a Phoenix rally a few weeks ago, he said he’d happily shut down the government to force a deal for Wall funding -- and rationalizing why it’s not being built.

Upon news of a potential deal with Chuck Schumer and Nancy Pelosi over DACA that wouldn’t include funding, Trump tweeted that The Wall “is already under construction in the form of new renovation of old and existing fences and walls.” Needless to say, none of his rally-goers thought they were cheering for routine maintenance at the border.

Despite the drama, The Wall is mostly beside the point when it comes to enforcement. Construction of a wall doesn’t even make the top three priorities for tightening up on illegal immigration. A mandatory e-verify system to discourage illegal hiring, an entry-exit system to track visitors, and local and state cooperation with the feds all are much more important. A border wall is powerless to stop visa overstays, which account for about half of illegal immigration, and it won’t reduce the jobs magnet that inevitably draws people here.

If Democrats were smart, they’d let Trump build whatever he wants on the border in exchange for massive concessions on other policies. But the Democratic base is too adamantly against The Wall, which it considers a symbol of exclusion and xenophobia, to make this negotiating strategy possible. Upon contact with the enemy, Trump will be lucky if The Wall ends up as much more than architectural plans and demonstration projects.

Rich Lowry is editor of The National Review.