“It would cost a lot less to train the military on the border and keep them primed for a southern invasion.”
Each approach has is pros and cons.
The kind of barrier they are looking to build now (30 foot tall bollards) costs a lot the year you put it in, but very little after that. Using more people, has costs far into the future, including retirement and disability payments.
Military units are budgeted into the various operational plans that the Pentagon develops. For example, if a land war breaks out in Korea, specific units, by name, are designated specific roles. If you want to task that unit with something else (like moving to the border, and assuming a guard mission), then you have to explicitly plan and resource to replace them, or backfill them with another limited resource when they do deploy.
Every year, during the budget drill, the Iron Majors in the Pentagon burn the midnight oil on endless “what if” drills, juggling the many variables to allocate resources. There is no free lunch - units that are not operationally justified (essential) have long since been squeezed out of existence.
It is in fact very expensive to develop and maintain military units, and it lessens their proficiency/combat effectiveness if you have them doing different missions (like border guard) for a long time.
In the short term, an active duty military unit can quickly bring a lot of capability to bear, but if it is going to be long term, you have to deliberately develop dedicated force structure to support the mission. Border Patrol is specialized in their task, without the competing pull of a combat mission. They are inherently better suited in that respect.
If we did pay the premium to develop active duty military units for border guard, there would be the added benefit of having their broader capability to potentially use them temporarily for other contingencies, like natural disasters, civil unrest or reinforcing a major war.
Bottom line: There is no free lunch, to use military units for border security long term. When you add all the lifecycle costs, they are actually quite expensive, per person.
Bollard style barrier on the other hand, is cheap to maintain, and allows the Border Patrol to channel the threat and concentrate their personnel, making them able to be more effective per person. Lots of people are tempted to hop the fence, because it looks (is) so easy. Eighteen foot bollards effectively deterred a great percentage of crossers where they were installed. Thirty foot bollards will deter all but the most fit and daring - almost everyone.
In the end, the most efficient use of resources to effectively control the border is a mix of personnel, equipment, technology, and infrastructure (barrier, roads, lighting and sensors).
Another option to reinforce the Border Patrol, could be the use of civilian contractors, like security guards as lookouts, or one of those companies that monitor home security systems - the kinds of support services that don’t require carrying a gun or laying hands on prisoners.
I was down there when the National Guard was down there and used as a force multiplier for the BP. Worked good. There were some hard chargers in these groups that we ran across. We would strike the illegals, day or night, and contain them with our force and bring in the BP or NG who would then flush them with their Oscar force. We had some real Keystone cop chases. The problem was we were 75-80 miles INSIDE our border.
This would not have to be done if you positioned military (NG, State Militia) on the BORDER facing south with proper ROEs to engage anything crossing the border until barrier is complete.
Why not?