In December, Edward led his army across the River Severn and advanced into the Welsh Marches, where the opposition forces had gathered. The coalition of Marcher Lords crumbled and the Mortimers surrendered to Edward, but Damory, Audley, and the Earl of Hereford marched north in January to join Lancaster, who had laid siege the king's castle at Tickhill. Bolstered by fresh reinforcements from the Marcher Lords, Edward pursued them, meeting Lancaster's army on 10 March at Burton-on-Trent. Lancaster, outnumbered, retreated without a fight, fleeing north. Andrew Harclay cornered Lancaster at the Battle of Boroughbridge, and captured the earl. Edward and Hugh the Younger met Lancaster at Pontefract Castle, where, after a summary trial, the earl was found guilty of treason and beheaded.