Complex sentence: Their sudden arrival contributed to an increase in criminal activities throughout the 1990s and 2000s (decade), but the Greek Police also had several victories, breaking up mafia organizations, arresting the 17 November terrorist group, and successfully organizing security for the 2004 Olympic Games.
Split into simpler sentences: Their sudden arrival contributed to an increase in criminal activities throughout the 1990s and 2000s. The Greek Police also had several victories, breaking up mafia organizations, arresting the 17 November terrorist group, and successfully organizing security for the 2004 Olympic Games.