Complex sentence: Beginning in 1954, Viet Minh sympathizers in the South were subject to escalating suppression by the Diem government, but by December 1960 the National Liberation Front of Southern Vietnam had been formed and soon rapidly achieved de facto control over large sections of the South Vietnamese countryside.
Split into simpler sentences: Beginning in 1954, Viet Minh sympathizers in the South were subject to escalating repression by the Diem government. By December 1960 the National Liberation Front of Southern Vietnam had been formed and rapidly achieved de facto control over large sections of the South Vietnamese countryside.