I completely agree with you that having unauthorized access to sensitive data can cause severe harm not only to the organization but also to people, nations, and even the ones who stole the information.
The example you provided about the young serviceman who leaked classified information from the Pentagon highlights the importance of having robust security measures in place. It’s alarming to think that someone who was trusted by the organization and had top-level security clearance could commit such a breach of security.
As you suggested, implementing measures such as firewalls, restricting the downloading and printing of sensitive material, prohibiting personal devices while at work, and security training can go a long way in mitigating the risks of a compromise from within. Additionally, I agree with your idea of checking people’s backpacks, coats, and briefcases before they leave a particular organization or government facility. It might seem like a bit of an inconvenience, but it’s a necessary measure to ensure that sensitive information doesn’t leave the premises.