On his first day in office, one of the orders that Trump issued declared a national emergency at the borders. It was accompanied by an executive order to the Department of Defense to engage in planning for the use of the military at the southern border. This is a complicated issue, with many nuances.
The best explanation I have seen about this topic is from Mark Nevitt, in the Just Security Blog. The piece is entitled
What Just Happened: Unpacking Exec Order on National Emergency at the Southern Border
This is an essential read!
My take? My initial reaction to this is the same as it was the last time around (2017). It’s eyewash for Trump with his base and a useless waste of money and, more important, the attention of the military, which is spread thin as it is.