Approximately 4,000–5,000 French, Breton and Genoese mercenaries (the largest field army ever assembled by Charles of Blois) laid siege to the town of La Roche-Derrien in the hope of luring Sir Thomas Dagworth, the commander of the only standing English field army in Brittany at the time, into an open pitched battle. When Dagworth's relief army, less than one-fourth the size of the French force, arrived at La Roche-Derrien they attacked the eastern (main) encampment and fell into the trap laid by Charles. Dagworth's main force was assailed with crossbow bolts from front and rear and after a short time Dagworth himself was forced to surrender. Charles, thinking he had won the battle and that Brittany was effectively his, lowered his guard. However a sortie from the town, composed mainly of townsfolk armed with axes and farming implements, came from behind Charles's lines. The archers and men-at-arms who remained from the initial assault now rallied with the town's garrison to cut down Charles' forces. Charles was forced to surrender and was taken for ransom.