Complex sentence: In December 1978, Vietnam invaded Cambodia and soon took over most of the country, establishing a pro-Vietnamese government; with the Khmer Rouge removed from power, the floodgates opened and Cambodians attempted to cross into Thailand in large numbers.
Split into simpler sentences: In December 1978, Vietnam invaded Cambodia, ousting the Khmer Rouge and establishing a pro-Vietnamese government. With this, the floodgates were opened and Cambodians attempted to cross into Thailand in large numbers.