Henry's speech is powerful. It lays out all the reasons, piece by piece, the colonists have been forced into a corner, and that outright war is all that is left. His rhetoric is flawless: he begins by giving full credit to his colleagues who have other opinions--something most of our politicians could study and benefit from today--then carefully lays out the reasons things have gone too far for simple diplomacy now: that a war is in the offing whether they choose to arm and fight or not.
His concession that hope is admirable, but can blind us to reality, is also powerful. It shows the President that he is listening and he understands. He follows this concession, however, with the undeniable point that it's ill-advised to let hope blind us to reality: the British are moving their navy against the colonists, and at this point, there's no real choice left beyond accepting the lock-down and oppression sure to follow or to seriously prepare for a war.
Were I sitting in that convention when he gave that speech, even had I originally been a Loyalist, I'd have been moved by his logic and his call for arms.