I have a very good idea of how effect such forces would be. The answer is not very effective.
It won't be from any lack of skills or determination.
The enemies have organized propaganda and firepower (Mexican Army, Police, and Cartel forces) to back their demands. They also have a substantial "fifth column" of Leftist supporters in this Country. We have indecisive political leaders at the State, County, and Municipal levels who will actually move to stop volunteer forces from doing anything effective.
That has to change before volunteers can be a decisive force. The Police and Military forces will need directions from their commanders to avoid confrontations with American volunteers. The political Authorities (or their replacements) will need to give those directions.
On the plus side, the Mexican forces will probably turn on each other at the first setback and destroy themselves. This invasion will disintegrate at the first real opposition.
We have not seen that yet. Let's not get too wrapped up in our own propaganda.
Some appropriate song lyrics to keep in mind about our politicians:
"And if you really want to know our view - you haven't done nothing" (Stevie Wonder)
“We have indecisive political leaders at the State, County, and Municipal levels who will actually move to stop volunteer forces from doing anything effective.”
Under present circumstances (Invasion force, act of war) such actions would constitute treason. Those individuals would thus warrant being shot for making such impediments that provide aid and comfort to the enemy - in a war situation.
We’ve already had incidents before where armed volunteer groups related in some cases to the Oathkeepers have kept watch on the border with coordination with State officials.
There were no problems.
If a conflict erupted? That did not happen.
I think there were some in AZ, MX “mules” with Oathkeepers.