No satisfactory explanation has ever been given of the reasons which induced Edward Bruce to agree to so one sided a bargain.  He had already invested the place for four months, there was no possibility of an army being collected in England for its relief for many months to come, and long ere this could arrive the garrison would have been starved into surrender.  By giving England a year to relieve the place he virtually challenged that country to put forth all its strength and held out an inducement to it to make that effort, which internal dissension had hitherto prevented.  The only feasible explanation is that Edward Bruce was weary of being kept inactive so long a time before the walls of the fortress which he was unable to capture, and that he made the arrangement from sheer impatience and thoughtlessness and without consideration of the storm which he was bringing upon Scotland.  Had it been otherwise he would surely have consulted the king before entering upon an agreement of such extreme importance.

Bruce, when he heard of this rash treaty, was highly displeased, but he nevertheless accepted the terms, and both parties began at once their preparations for the crowning struggle of the war.  The English saw that now or never must they crush out the movement which, step by step, had wrested from them all the conquests which had been won with such vast effort under Edward I; while Bruce saw that a defeat would entail the loss of all that he had struggled for and won during so many years.

King Edward issued summonses to the whole of the barons of England and Wales to meet him at Berwick by the 11th of June with all their feudal following, while the sheriffs of the various counties and towns were called upon to supply 27,000 foot soldiers.  The English of the settlements in Ireland were also summoned, besides O’Connor, Prince of Connaught, and twenty-five other native Irish chiefs, with their following, all of whom were to be under the command of Richard de Burgh, Earl of Ulster.

The Prince Bishop of Constance was requested to furnish a body of mounted crossbowmen.  A royal fleet of twenty-three vessels was appointed to assemble for the purpose of operating on the east coast, while the seaports were commanded to fit out another fleet of thirty vessels.  A third fleet was ordered to assemble in the west, which John of Lorne was appointed to command under the title of High Admiral of the Western Fleet of England.  From Aquitaine and the French possessions the vassals were called upon to attend with their men-at-arms, and many knights from France, Gascony, and Germany took part in the enterprise.

Thus, at the appointed time over 100,000 men assembled at Berwick, of whom 40,000 were men-at-arms, and the rest archers and pikemen.  For the great armament the most ample arrangements were made in the way of warlike stores, provisions, tents, and means of transport, together with the necessary workmen, artificers, and attendants.