This work shows that the entry into the English Civil War of the Scots on the side of Parliament radically changed the balance of power in the North of England. The Royalists were forced onto the defensive and the Marquis of Newcastle found his army besieged in York. Prince Rupert proposed a bold plan to relieve York. He succeeded beyond all measure and entered York. Rupert then decided to march out to attack his enemies. After a desperately hard-fought battle the Royalist army broke. The loss of the North had decisively tilted the balance of military power in favour of Parliament.