Complex sentence: Most of Bentley's contacts were in what prosecutors and historians would later call the "Silvermaster group", a network of spies centered around Nathan Gregory Silvermaster that would become one of the most important Soviet espionage operations in the United States.
Split into simpler sentences: Most of Bentley's contacts were in what prosecutors and historians would later call the "Silvermaster group", a network of spies centered around Nathan Gregory Silvermaster. This network would become one of the most important Soviet espionage operations in the United States.