Among the many ill-advised components of Donald Trump's "immigration plan" is his instance on building a wall on the southwestern border with Mexico (Pg. 1-2 of his plan). Never mind that much of the U.S. Mexico's 1,954 mile U.S.-Mexico border is already fortified and guarded, but pledging to beef up the border with either physical barriers, more border patrol personnel, or newfangled technology and equipment is a favorite promise by both Democratic and Republican politicians running for public office. And now that Trump has called dibs on a wall on the southwestern border, Scott Walker is talking about a fence on the northern border.
I've beat this dead horse many times, but here I go again: The main reason building a wall is a waste of money and will be ineffective in curbing undocumented immigration is that there is more than one way into the U.S. Multiple sources, including the nonpartisan Migration Policy Institute, agree that at least 40% of the undocumented population currently in the U.S. did not enter by surreptitiously crossing the southwestern border, but instead they arrived on a valid temporary non-immigrant visa (think of tourist, student, and temporary work visas), overstayed, and didn't go home. When their visas expired; these people (known as "visa overstays") became part of the 11 million undocumented population.
Therefore, fortifying or building a wall and amassing border patrol agents, drones, cameras, and what have you on the southwestern border is tantamount to you (before leaving for work each morning) dutifully double deadbolt locking your front door, securing your back door with a twisty tie from the supermarket, and leaving the roof off of your house.
The "build a wall" proposal makes no good sense, yet neither Trump nor the dozens of politicians before him who have gone down this well tread path are fools. As Peter Andreas explained in Border Games--Policing the U.S- Mexico Divide public policies don't actually have to work to have political utility to the proposer. The move allows the proposer to look tough on undocumented immigration without actually doing much about it.