This speech was given by Emperor Constantine XI of Constantinople, on May 28, 1453, given to his advisors, troops, and whoever else was there to listen. Edward Gibbon referred to it as "the funeral oration of the Roman Empire", in The Decline and Fall of the Roman Empire and so I titled it as such.
Here it is, according to the account of Leonardo of Chios, the Latin archbishop of Lesbos.
"Gentlemen, illustrious captains of the army, and our most Christian comrades in arms: we now see the hour of battle approaching. I have therefore elected to assemble you here to make it clear that you must stand together with firmer resolution than ever. You have always fought with glory against the enemies of Christ. Now the defence of your fatherland and of the city known the world over, which the infidel and evil Turks have been besieging for two and fifty days, is committed to your lofty spirits.
Be not afraid because its walls have been worn down by the enemy's battering. For your strength lies in the protection of God and you must show it with your arms quivering and your swords brandished against the enemy. I know that this undisciplined mob will, as is their custom, rush upon you with loud cries and ceaseless volleys of arrows. These will do you no bodily harm, for I see that you are well covered in armour. They will strike the walls, our breastplates and our shields. So do not imitate the Romans who, when the Carthaginians went into battle against them, allowed their cavalry to be terrified by the fearsome sight and sound of elephants.
In this battle you must stand firm and have no fear, no thought of flight, but be inspired to resist with ever more herculean strength. Animals may run away from animals. But you are men, men of stout heart, and you will hold at bay these dumb brutes, thrusting your spears and swords into them, so that they will know that they are fighting not against their own kind but against the masters of animals.
You are aware that the impious and infidel enemy has disturbed the peace unjustly. He has violated the oath and treaty that he made with us; he has slaughtered our farmers at harvest time; he has erected a fortress on the Propontis as it were to devour the Christians; he has encircled Galata under a pretence of peace.
Now he threatens to capture the city of Constantine the Great, your fatherland, the place of ready refuge for all Christians, the guardian of all Greeks, and to profane its holy shrines of God by turning them into stables for fits horses. Oh my lords, my brothers, my sons, the everlasting honour of Christians is in your hands.
You men of Genoa, men of courage and famous for your infinite victories, you who have always protected this city, your mother, in many a conflict with the Turks, show now your prowess and your aggressive spirit toward them with manly vigour.
You men of Venice, most valiant heroes, whose swords have many a time made Turkish blood to flow and who in our time have sent so many ships, so many infidel souls to the depths under the command of Loredano, the most excellent captain of our fleet, you who have adorned this city as if it were your own with fine, outstanding men, lift high your spirits now for battle.
You, my comrades in arms, obey the commands of your leaders in the knowledge that this is the day of your glory -- a day on which, if you shed but a drop of blood, you will win for yourselves crowns of martyrdom and eternal fame."
I found this at rumkatkilise.org/constantinexi