Complex sentence: The city was sacked in 1500 AD when King John of the Kalmar Union attempted to conquer the republic--his forces were self-assured and sloppy, and were routed by a force of poor arms and far inferior numbers in the Battle of Hemmingstedt.
Split into simpler sentences: The city was sacked in 1500 AD when King John of the Kalmar Union attempted to conquer the republic. His forces were routed by a force with poor arms and inferior numbers in the Battle of Hemmingstedt.