What makes you think limiting the intruders to this country wouldn't help? How did the terrorists get into Paris? They weren't French, you know.
Terrorist get into Paris? They crossed the border.
We are talking about building a wall south of our border. Our border to the south is not as easy to penetrate than what people thought. It's dotted with thousands of sensors with over 21,000 of border patrol. Between 2013 and 2014 total of almost 700,000 was apprehended. So far there are no middle eastern people. If they come which I think they are already here IMO they will or came from the north border or east.
Border with Canada is wide open. There are check points in major highways but there is nobody to stop them when crossing the open farms, mountain etc...
To the east there are middle easterners working as seaman and as one previously mentioned. We do not even check 1/10 of shipping containers.... So what good is the fence?