Complex sentence: The Manhattan District organized its own counterintelligence which gradually grew in size and scope, but strict security measures failed to prevent the Soviets from conducting a successful espionage program that stole some of its most important secrets.
Split into simpler sentences: The Manhattan District organized its own counterintelligence which gradually grew in size and scope. Strict security measures failed to prevent the Soviets from conducting a successful espionage program that stole some of its most important secrets.