There is no way the Border Patrol can contain/detain 3-6 thousand people.
“No way BP can detain 3-6 thousand”
Especially if they don’t know where to set up.
The military can set up tent cities with life support, and then surge to wherever they cross, to set up cattle chutes to bring them into line for tagging, and put them on the bus.
I worked JTF-6 in the 80’s and 90’s south of our border....... We did “not” have powers to arrest but we “did” have the powers to detain for our own protection. Armed and detaining a bad guy with force..... or armed and arresting a bad guy with force . It got done, troops on the border will do the job.....