Complex sentence: In both incidents, the North Koreans were the first to open fire, though in the 1999 incident the South Koreans escalated matters by initiating a campaign of boat'bumping' in order to stop what the South saw as a violation of its maritime borders.
Split into simpler sentences: In both incidents, the North Koreans were the first to open fire. In the 1999 incident the South Koreans escalated matters by initiating a campaign of boat'bumping' in order to stop what the South saw as a violation of its maritime borders.