Complex sentence: Southwest debuted the international reservation system on January 27,2014 followed by the first international flights which began on July 1,2014 to Aruba (AUA), Montego Bay, Jamaica (MBJ) and Nassau, Bahamas (NAS).
Split into simpler sentences: Southwest debuted the international reservation system on January 27,2014 when its first international flights went on sale. This was followed by the first international flights which began on July 1,2014 to Aruba (AUA), Montego Bay, Jamaica (MBJ) and Nassau, Bahamas (NAS).