Contracts exist, not fences. And much of the land is not owned by the government, so nothing can legally be built on it. It's doubtful that the Biden admin will move to seize that land by eminent domain (something the Trump admin didn't try) and declines to negotiate with the land owners, that part of the fence will never be built.
It's just another example of Trump trying to make things difficult for his successor as he heads out the door. |