Complex sentence: The Polish Operation of the NKVD in 1937--1938 was a Soviet Great Purge-era mass operation against purported Polish agents in the Soviet Union, explicitly ordered against Polish spies, but interpreted by the NKVD as relating to "absolutely all Poles".
Split into simpler sentences: The Polish Operation of the NKVD in 1937--1938 was a mass operation by the NKVD against purported Polish agents in the Soviet Union during the period of the Great Purge. It was explicitly ordered against Polish spies, but interpreted by the NKVD as relating to "absolutely all Poles".