The Manchester concert bombing is an example of the limits of defensive security.
There was security at the concert, but the bomber apparently didn't try to get into the venue, instead blowing himself up in an entrance foyer area as concertgoers flooded out of the arena. Prime Minister May said the attacker had deliberately chosen "his time and place to cause maximum carnage" in the young crowd.
No matter how you play defense, no matter where you put the security perimeter, you can't avoid creating a choke-point that is itself a soft target for an attack. Security waiting lines, entrances and exits, are impossible to secure by their very nature.
This harsh reality is why relying on defensive measures against terrorism is a fool's game. We can only win by going on the offense.
