Complex sentence: As one of the main military leaders on the border to Scotland, Harclay became frustrated with Edward II's inactivity, particularly the humiliating English defeat at the Battle of Old Byland on 14 October 1322, which made it clear that the war could not be won.
Split into simpler sentences: As one of the main military leaders on the border to Scotland, Harclay became frustrated with Edward II's inactivity. Particularly the humiliating English defeat at the Battle of Old Byland on 14 October 1322 made it clear that the war could not be won.