>>>A bit of a tangent, but how is the US/Mexico border a "defense" issue?
Border security is THE most basic, fundamental responsibility of national defense. The US has just spent so frickin' long with benign powers on its land borders, and so focused on global expeditionary operations, that its border security efforts have atrophied. That said, national defense is now an over-arching term that includes a mix of law enforcement and military responsibilities and tasks. Note that securing our airspace "borders" largely falls onto Combat Air Patrols conducted by the various state Air National Guards, typically flying F-16s or F-15s.
>>>I am unaware of any Latin American terrorist groups threatening the USA.
Maybe not Latin American ones, but Al Qaeda just called for renewed attacks on the US in the wake of the 9/11 anniversary.[1] We KNOW that illegal aliens are crossing into the US traveling from as far as west/central Africa. [2] West Africa already has an Islamic terrorism problem.[3] From a risk assessment/risk management perspective, it is wise to allocate resources to reduce the chance of AQ or ISIS agents sneaking across our southern border, which leaks like a sieve.
>>>If the US/Mexico border is a defense issue, then it should be entirely militarized?
I think most major countries handle it as a national-level law enforcement responsibility, but sometimes there are still significant military forces involved.
As a point of comparison, China uses Army forces for border security, in addition to other agencies.[4] Russia's primary border force is a law-enforcement organization.[5]
[1]https://www.zerohedge.com/geopolitical/al-qaeda-chief-issues...
[2]https://www.zerohedge.com/news/2019-06-02/watch-first-large-...
[3]https://en.wikipedia.org/wiki/Boko_Haram
[4]https://china-defense.blogspot.com/2017/09/the-second-line-p...
[5]https://en.wikipedia.org/wiki/Border_Service_of_the_Federal_...