Complex sentence: When Remus and Romulus became adults, they decided to establish a city; however, they could not decide who would rule it, Romulus and Remus had a war to see who would become ruler of their new city.
Split into simpler sentences: When Remus and Romulus became adults, they decided to establish a city. However, they could not decide who would rule it, Romulus and Remus had a war to see who would become ruler of their new city.