 Before the end of the year , Gaveston had returned to England , and civil war appeared imminent . In May 1312 , Gaveston was taken captive by the Earl of Pembroke , but Warwick and Lancaster had him abducted and executed after a mock trial . This affront to Pembroke ’ s honour drove him irrevocably into the camp of the king , and thereby split the opposition . The brutality of the act initially drove Lancaster and his adherents away from the centre of power , but the Battle of Bannockburn , in June 1314 , returned the initiative . Edward was humiliated by his disastrous defeat , while Lancaster and Warwick had not taken part in the campaign , claiming that it was carried out without the consent of the baronage , and as such in defiance of the Ordinances . 
