When crime goes up, people lock their doors. When terrorist strike or mass refugees, countries close their borders like they do in Europe and implement checkpoints on the roads.
If it is a home grown threat, who you gonna call? Phone NSA.
Actually, read something sort of funny the other day, just for some comic relief.
Some wise ass remarked that the same people criticizing Trump for this Great American Wall idea to separate Americans from Mexicans, are the same people who remain silent on the Isreali Wall separating the Jews and Arabs. It's only funny because it likely true.
Walls have been built since ancient times. Like moats with castles and watch towers.
