For better or worse, defense of the homeland is not accomplished by literally standing at the US border waiting for someone to step over the line, but by working to neutralize threats and prevent their emergence abroad.
Fair enough, I'll admit the line is a little fuzzy. But there's clearly a point where we've gone way beyond any reasonable notion of "defense" and are engaging in offense, oppression, empire building, etc. And I'd argue that we're way over that line these days.
