In late 1425, Jean, Duke of Brittany, had switched his allegiance from the English to Charles the dauphin. In retaliation, Sir Thomas Rempston invaded the duchy with a small army in January 1426, penetrating to the capital, Rennes, before falling back to St. James-de-Beuvron on the Norman frontier. The duke of Brittany's brother, Arthur de Richemont, newly made constable of France, rushed to his brother's aid. Richemont hastily levied an army across Brittany in February and gathered his forces in Antrain. The newly assembled Breton force first captured Pontorson, executing all the surviving English defenders and entirely destroying the wall after seizing the city. By the end of February, Richemont's army then marched on St. James. Rempston was heavily outnumbered, with 600 men to Richemont's feudal horde of 16,000.

Richemont was reluctant to launch a full assault with troops of such poor quality. After holding a council of war with his officers, he decided to assault the walls through the two breaches. On the 6th of March the French attacked in force. All day Rempston's troops held the breaches, but there was no let-up in the constable's assault. The English defenders capitalized on a panic that ensued among the largely ill-trained Breton militia to inflict heavy losses on the fleeing Breton troops. During the chaotic retreat, hundreds of men drowned crossing the nearby river while many others fell to the deadly bolts of the defenders' crossbows.