I don't harbor any illusions that anyone's going to read that entire article about how the American Southwest came to need a fence at the particular location where it's thought to be needed.  Instead, I'll give you a brief quote by a great American to summarize the US foreign policy that got us here.

After the filming of The Comancheros, great  American, John Wayne was asked during an interview whether he had any problem with the fact we'd taken all the land away from the Indians by force of arms, then 'given' tiny pieces of it back to them.

"When Europeans got here," Wayne answered, "the Indians were selfishly hoarding the entire continent."

Europeans took care of that little problem. 

Then Mexico attempted to selfishly hoard the entire (now) southwestern US.  Which rightfully weeed-off the good Amercuns, forcing them to take it away from Mexico by armed force.

Now the former selfish hoarders of the Amercun southwest are breaking US laws by re-entering the country they had to have taken from them because of their selfishness. 

A lot of Amercuns don't like that.  So the US government has to do something to try to convince them they're doing something. 

A fence.

Meaningless gestures have always worked well, thus far.

Meanwhile, the US has discovered Moslem people are selfishly hoarding land that has a lot of petroleum beneath it.  Question is whether to send them somewhere else when we have to punish them for selfishly hoarding that land, or just figure out a justification for killing them all.

Just my rad-lib viewpoint, supported by actual history, rather than patriotic myth.

J