Complex sentence: The Romans had been fighting with the Latin and Campanian peoples against the Samnites in the First Samnite War; when the Romans withdrew from the war, the Latins kept fighting besides the Campanians, while Rome joined the Samnites to attack the Latins.
Split into simpler sentences: The Romans had been fighting with the Latin and Campanian peoples against the Samnites in the First Samnite War; when the Romans withdrew from the war. The Latins continued fighting besides the Campanians, while Rome joined the Samnites to attack the Latins.