Final answer:
Marcus Junius Brutus and Gaius Cassius Longinus are definitively known as the leaders of the conspiracy that resulted in Julius Caesar's assassination on the Ides of March, driven by their aim to protect the Roman Republic from autocracy.
Step-by-step explanation:
Individuals who are definitively known to be part of the conspiracy to assassinate Julius Caesar include Marcus Junius Brutus and Gaius Cassius Longinus. These two prominent senators led the plot against Caesar due to their strong belief in the Republic and their fear of Caesar's growing power, seeing him as a potential tyrant.
The background to the conspiracy involves Caesar being appointed as dictator for life, which left many Romans, who were wary of autocracy, uneasy. The resentment among the senators grew, leading to the conspiracy that culminated in Caesar's assassination on the Ides of March (March 15) 44 BCE.