What does Cassius confide to Messala before the battle?

In Shakespeare's Julius Caesar, before the battle Cassius reveals to Messala that it is his birthday.  Cassius says:

...Messala,

This is my birthday:  as this very day

Was Cassius born.  Give me thy hand, Messala:

Be thou my witness that against my will

(As Pompey was) am I compelled to set

Upon one battle all our liberties.  (Act 5.1.70-75)

Cassius is regretting that he has to set defeat or victory upon one, single battle.  He compares himself to Pompey, who had to do the same against Caesar, and lost.  He asks Messala to be his witness that he is doing this against his will.

Eventually, of course, Cassius's birthday will also become the day of his death.  His fear becomes reality as the one, single battle is lost, as are all their "liberties," including Cassius's life.