What Good is Overseas Defense When Millions of Invaders Cross the Border Every Year Do You Now See Disconnect?

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Out of the two or three million illegal aliens entering the U. S. every year (at the current rate), how many of them do you suppose have malevolent intent, to smuggle, or thieve the welfare and hospital systems, or commit violent crimes for robbery or just for the thrill of it, or to avoid criminal justice in their home countries, maybe half of those entering in one or more of those categories? So if the U. S. military is for national defense, why are they not keeping out those millions by building walls and patrolling them?