Society cannot invest enough resources to protect everything, everywhere, all the time.
Someone wanting to set off a bomb in Manhattan to kill scores of people can do it.
And reducing terrorism has nothing to do with access control or how thick you make the concrete wall. The process begins, after reasonable physical security precautions have been taken and preparations have been made, with the identification of potential threat groups. The vital first step is to identify the target groups and understand their motives and ability to carry out acts. Terrorists are either international or domestic. Terrorist acts instigated or supported by other nations or foreign interests are relatively uncommon in the United States.