If ISIS has its eye on hitting the US, let them try. Let us remain vigilant and stop it before it starts. And if ISIS or some other terrorists organization does succeed, let them face the full power of the US military in short order, no holds barred. As for who is crossing our southern border, I highly doubt terrorists cells are coming into the US this way. Best estimates have us catching about 30% of all crossers. An additional 10%, minimum, of crossers die in the desert during the trip. Hard to know full numbers, because many bodies decompose long before they are discovered. If you are a terrorists organization wanting to get a cell in the US composed of 20 men, having them enter though Mexico isn't a part of your plans. Statistically, 6 out of the 20 would get caught, and at least 2 wouldn't survive the trip, which would put a serious dent into the success of the operation.