Complex sentence: The conspirators'attempt to use the opportunity to restore the Roman Republic was thwarted: on the day of the assassination of Caligula, the Praetorian Guard declared Caligula's uncle, Claudius, the next Roman emperor.
Split into simpler sentences: The conspirators'attempt to use the opportunity to restore the Roman Republic was thwarted, however. On the day of the assassination of Caligula, the Praetorians declared Caligula's uncle, Claudius the next Roman emperor.