Jhesus-Maria, King of England, and you, Duke of Bedford, who call yourself regent of the Kingdom of France, you, Guillaume de la Poule, count of Suffort, Jean, sire of Talbot, and you, Thomas, sire of Scales, who call yourselves lieutenants of the Duke of Bedford, acknowledge the summons of the King of Heaven. Render to the Maid here sent by God the King of Heaven, the keys of all the good towns which you have taken and violated in France. She is here come by God’s will to reclaim the blood royal. She is very ready to make peace, if you will acknowledge her to be right, provided that France you render, and pay for having held it. And you, archers, companions of war, men-at-arms and others who are before the town of Orleans, go away into your own country, by God. And if so be not done, expect news of the Maid who will come to see you shortly, to your very great injury. King of England, if you do not so, I am chief-in-war and in whatever place I attain your people in France, I will make them quit it willy-nilly. And if they will not obey, I will have them all slain; I am here sent by God, the King of Heaven, body for body, to drive you out of all France … (Written this Tuesday of Holy Week, March 22, 1429.)
Joan of Arc, the Maid of Orleans, sends a formal letter of summons to the English upon the siege of Orleans. (I post this once a year on this date at all the places I blog.)
Update: Over at Volokh Conspiracy, my friend and co-blogger Ilya Somin offers an example of a much, much shorter way to declare war, courtesy the 10th century Prince of Kiev.