Harclay's most prominent achievement came with the Battle of Boroughbridge in 1322. The battle was the culmination of an ongoing struggle between King Edward II and his most powerful earl, Thomas of Lancaster. The conflict stemmed from disagreement over the handling of the war with Scotland; Lancaster, and many others, found the king's war effort lacking. After failing to incite an insurrection against the king, Lancaster was in March 1322 fleeing northwards from the royal army. Meanwhile, Harclay, as sheriff of Cumberland, was ordered by the king to levy the forces of the northern counties of Cumberland and Westmorland, and move south. His orders were to meet up with the royal army, but while stopping at the town of Ripon in Yorkshire, he received intelligence that Lancaster would arrive at nearby Boroughbridge the next day. Harclay decided to take the initiative, and occupy the bridge that would prevent Lancaster's passage across the River Ure.