Complex sentence: The Samnites received a report that the Romans had been besieged by the Etruscans and decided to confront Gaius Marcius and that, if he avoided battle, they would march to Etruria via the lands of the Marsi and the Sabines.
Split into simpler sentences: The Samnites received a report that the Romans had been besieged by the Etruscans and had decided to confront Gaius Marcius. The report also indicated that, if Gaius Marcius avoided battle, the Samnites would march to Etruria via the lands of the Marsi and the Sabines.