Obviously the two missions are not mutually exclusive, and securing the southern border would cost a tiny fraction of our defense budget.
The question is whether we need to maintain treaty alliances throughout the world or retreat into isolation. If the answer is no, then we can shrink the defense budget by well over 50% by eliminating bases and retiring our carrier fleets.
If we continue along the path of Slow Joe and his cronies, we won’t have much of a choice of allies left. They are pi**ing everyone off. Saudis, UAE, and Egypt are all joining BRICS.