It was the border wall that didn’t get built.

In 2006, Boeing and a team of other companies won a federal contract to construct a wall to protect the United States border with Mexico, which stretches roughly 2,000 miles, from California to Texas.

Five years and about $1 billion later, the government threw in the towel. Costs had ballooned, and the surveillance systems suffered from technical difficulties. Nearly all of the money had been spent on just 53 miles of the border in Arizona.

The project was a loss for taxpayers. But for contractors, it was a big win.

Today, as President Trump declares his intention to move forward with plans to build a barrier along the Mexican border, many of the details remain little more than a guessing game. Does Mr. Trump intend to build miles of concrete blocks, or fencing? Could parts of the wall be virtual, using technology like cameras and sensors to monitor the border, or be manned by drones? Will Mexico, as Mr. Trump has promised repeatedly, pay for it?

There is no doubt that if the United States moves ahead with plans for an ambitious border wall — one of the biggest infrastructure projects in decades, perhaps running in the tens of billions of dollars — it will be a boon for contractors.