No question that the our Southern border is a national security issue.
And our Northern border as well.
I agree, the southern border is the front of a foreign civilian invasion (as opposed to a military invasion) and the government has a Constitutional obligation to act.
"The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence."
US Constitution Article IV Section 4