How does 1,080 miles effectively seal off the whole US-Mexico border?
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I think they are adding that to the existing border fencing.
So far it seems like what they are doing is replacing lots of usually pathetic attempts at barriers, and counting that as miles of “walls”.
Now, if they replaced those weak barriers and additionally added 1,080 miles of new wall—that indeed would go a long way to securing the border in that sense.