only the first step in the campaign to recover the French possessions that he felt belonged to the English crown. Agincourt also held out the promise that Henry's pretensions to the French throne might be realised. Diplomacy Command of the sea was secured by driving the Genoese allies of the French out of the English Channel. While Henry was occupied with peace negotiations in 1416, a French and Genoese fleet surrounded the harbour at the English-garrisoned Harfleur. A French land force also besieged the town. In March 1416 a raiding force of soldiers under the Earl of Dorset, Thomas Beaufort, was attacked and narrowly escaped defeat at the Battle of Valmont after a counterattack by the garrison of Harfleur. To relieve the town, Henry sent his brother, John, Duke of Bedford, who raised a fleet and set sail from Beachy Head on 14 August. The Franco-Genoese fleet was defeated the following day after the gruelling seven-hour Battle of the Seine and Harfleur was relieved. Diplomacy successfully detached Emperor Sigismund from supporting France, and the Treaty of Canterbury—also signed in August 1416—confirmed a short-lived alliance between England and the Holy Roman Empire. 1417–20 campaign and 1421 campaign With those two potential enemies gone, and after two years of patient preparation following the Battle of Agincourt, Henry renewed the war on a larger scale in 1417. After taking Caen, he quickly conquered Lower Normandy and Rouen was cut off from Paris and besieged. This siege has cast an even darker shadow on the reputation of the king, along with his order to slay the French prisoners at Agincourt. Rouen, starving and unable to support the women and children of the town, forced them out through the gates believing that Henry would allow them to pass through his army unmolested. However, Henry refused to allow this, and the expelled women and children died of starvation in the ditches surrounding the town. The French were paralysed by the disputes between Burgundians and Armagnacs. Henry skilfully played one against the other without relaxing his warlike approach. In January 1419, Rouen fell. Those Norman French who had resisted were severely punished: Alain Blanchard, who had hanged English prisoners from the walls of Rouen, was summarily executed; Robert de Livet, Canon of Rouen, who had excommunicated the English king, was packed off to England and imprisoned for five years. By August, the English were outside the walls