The U.S. has awesome walls: the Atlantic and Pacific oceans. They saved our butt numerous times in WW2, and arguably are the reason that we're a global superpower today.
Sure, but today all it takes is a few people determined to kill themself - with all the technology that is available today, the power of a small dedicated group is vastly bigger today that it was 65 years ago, and it is only likely that it will get even bigger.