There is no single security solution that will stop a determined attacker. The best options are comprised of multiple layers; in this case those layers were adequate physical security design and controls, a properly trained guard force, and procedures designed to address the known threats.
When it works at its best, security will keep the threats at the perimeter and protect those inside. At a minimum, it should delay an attacker long enough to react and try and mitigate the threat, or get out of harm's way.
When Security Measures Work | Stratfor