The border situation is a political problem. It would take a lot of divisions to completely seal off the border. The total force of the US Army is only 12 divisions.
There's too many people coming across to control the border. As long as illegals are given santuary in the 'santuary cities' a cuople hundred thousand troops guarding 2000 miles of border is not enough. Due the month on 200,000/2000 is 100 troops per mile. It takes 4 men to cover a week of 168 hours so when you factor in leave and support troops you are left with something like 15 men per mile.