Final answer:
Cassius warns the Romans about a conspiracy against Caesar on the Ides of March.
Step-by-step explanation:
In Shakespeare's play Julius Caesar, Cassius warns the Romans about a conspiracy against Caesar. Along with Marcus Junius Brutus, Cassius conspires to assassinate Caesar on the Ides of March, which is March 15th. This warning is an important plot point in the play and sets the stage for the events that follow.