You are an intelligence analyst for the FBI's Counterintelligence Division tasked with researching and producing a case study on a major espionage case from the past to help identify anomalies that could indicate future espionage. You have been assigned to write a 750-word case study on Robert Hanssen, Aldrich Ames, Ana Montes, or John Walker and address who was involved, when and where the espionage took place, what information was compromised, how it was obtained and shared, why the spy acted, lessons learned, and the case's impact.