The primary duty of the secret police in Stalin's Soviet Union, known as the NKVD, was to maintain state security and suppress dissent through surveillance, intimidation, and repression. They were responsible for arresting and eliminating perceived enemies of the Communist Party, conducting purges, and managing labor camps. The NKVD played a critical role in enforcing Stalin's policies, including the Great Terror, where millions of people were imprisoned, executed, or exiled. Their actions instilled fear and ensured compliance with the regime, deeply impacting Soviet society and governance.