Complex sentence: Duncan Chaplin Lee, descendant of Confederate General Robert E. Lee and protégé and trusted aide to Office of Strategic Services chief William J. Donovan, who became the NKVD's most senior source in American intelligence.
Split into simpler sentences: Duncan Chaplin Lee, descendant of Confederate General Robert E. Lee. Lee was a protégé and trusted aide to Office of Strategic Services chief William J. Donovan, and became the NKVD's most senior source in American intelligence.