Complex sentence: Up until 295 BC, the Samnites and the Kelts had been Rome's chief rivals, but that year, at the Battle of Sentinum, the Romans defeated the combined armies of the Samnites and the Kelts.
Split into simpler sentences: Up until 295 BC, the Samnites and the Kelts had been Rome's chief rivals. That year, at the Battle of Sentinum, the Romans defeated the combined armies of the Samnites and the Kelts.