Complex sentence: Afraid that the same fate might occur to them, the neighbouring Boii joined the Etruscans in a rebellion, but their combined force was defeated at the Battle of Lake Vadimo in the same year.
Split into simpler sentences: Afraid that the same fate might occur to them, the neighbouring Boii joined the Etruscans in a rebellion. Their combined force was defeated at the Battle of Lake Vadimo in the same year.