"Exactly how can we know this but not be able to stop them? "
Exactly how can we stop them? Israel has a border that is a computer sensor fence, ... The Arabs still smuggle over the border.
Controlling terrorism and illegal immigration is a matter of degree and quantity. If we can stop 99 percent of them, then we have succeeded. If we can stop 99 out of 100 nuclear backpacks, then the country can survive.
In answer to the question, "Exactly how can we know this but not be able to stop them? " here is part of the answer: "Sheriff D'Wayne Jernigan, of Del Rio, Texas, last month publicly blasted the dubious policy after being ordered by the Homeland Security Department to release 17 captured Brazilians before they had been interrogated." "Catch and release" is for environmentally conscious fishermen.
While I do not believe that we can "seal" the border, it is clear we can do more. Catch and release is stupid, let the dope smokers out of jail and put the illegal immigrants in. Minimum sentance 3 months. As it is there is no cost to being caught, other than a free busride home.
Bus should stop at the local pokey, illegal what a concept. You and I agree on much.