Cassius is right about Mark Antony all along. When the conspirators are planning the assassination of Julius Caesar, Cassius says:
I think it is not meet
Mark Antony, so well beloved of Caesar,
Should outlive Caesar. We shall find of him
A shrewd contriver. (II.1)
But Brutus overrules him, claiming that they must not seem too bloody if they want to be accepted as patriots and protectors of the Republic.
After Caesar is assassinated, Antony asks permission to display Caesar's body and speak in his funeral. Cassius is horrified when Brutus generously gives Antony permission to do so. Cassius says:
You know not what you do. Do not consent
That Antony speak in his funeral.
Know you how much the people may be moved
By that which he shall utter. (III.1)
But Brutus overrules him again. Brutus is a more noble man than Cassius, but Cassius is more worldly wise. Antony's speech turns the Roman citizens against the assassins and forces them to flee the city. This is the turning point in the play and the beginning of the end for Brutus and Cassius.