<human>:  Why did Stalin form the NKVD?
<bot>: The NKVD was formed in 1936 as a successor to the Cheka, which was dissolved in 1929. The Cheka was responsible for the mass arrests and executions of political enemies during the Russian Civil War. The NKVD was established as a security force for the Soviet Union, tasked with protecting the Soviet leadership and carrying out secret police activities. The NKVD was dissolved in 1946.