Complex sentence: In 1940 when Soviet Union occupied Lithuania and formed a Soviet administrative system, the older one (with counties) remained in force for some time but in 1950 the counties were abolished and entire area divided into smaller units-districts.
Split into simpler sentences: In 1940 when Soviet Union occupied Lithuania and formed a Soviet administrative system, the older one (with counties) remained in force for some time. But in 1950 the counties were abolished and entire area divided into smaller units-districts.