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 counter-attack 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.