During the earlier part of the year 1643 the military position of Charles was greatly superior to that of the parliament. Essex was inactive near Oxford; in the west Sir Ralph Hopton had won a series of victories, and in the north Newcastle defeated the Fairfaxes at Adwalton Moor, and all Yorkshire except Hull was in his hands. It seemed likely that the whole of the north would be laid open and the royalists be able to march upon London and join Charles and Hopton there. This stroke, which would most probably have given the victory to the king, was prevented by the "Eastern Association," a union of Norfolk, Suffolk, Essex, Cambridgeshire and Hertfordshire, constituted in December 1642 and augmented in 1643 by Huntingdonshire and Lincolnshire, of which Cromwell was the leading spirit. His zeal and energy met everywhere with conspicuous success. In January 1643 he seized the royalist high sheriff of Hertfordshire in the act of proclaiming the king's commission of array at St Albans; in February he was at Cambridge taking measures for the defence of the town; in March suppressing royalist risings at Lowestoft and Lynn; in April those of Huntingdon, when he also recaptured Crowland from the king's party. In May he defeated a greatly superior royalist force at Grantham, proceeding afterwards to Nottingham in accordance with Essex's plan of penetrating into Yorkshire to relieve the Fairfaxes; where, however, difficulties, arising from jealousies between the officers, and the treachery of John Hotham, whose arrest Cromwell was instrumental in effecting, obliged him to retire again to the association, leaving the Fairfaxes to be defeated at Adwalton Moor. He showed extraordinary energy, resource and military talent in stemming the advance of the royalists, who now followed up their victories by advancing into the association; he defeated them at Gainsborough on the 28th of July, and managed a masterly retreat before overwhelming numbers to Lincoln, while the victory on the 11th of October at Winceby finally secured the association, and maintained the wedge which prevented the junction of the royalists in the north with the king in the south.
One great source of Cromwell's strength was the military reforms he had initiated. At Edgehill he had observed the inferiority of the parliamentary to the royalist horse, composed as it was of soldiers of fortune and the dregs of the populace. "Do you think," he had said, "that the spirits of such base, mean fellows will ever be able to encounter gentlemen that have honour and courage and resolution in them? You must get men of a spirit that is likely to go as far as gentlemen will go or you will be beaten still." The royalists were fighting for a great cause. To succeed the parliamentary soldiers must also be inspired by some great principle, and this was now found in religion. Cromwell chose his own troops, both officers and privates, from the "religious men," who fought not for pay or for adventure, but for their faith. He declared, when answering a complaint that a certain captain in his regiment was a better preacher than fighter, that he who prayed best would fight best, and that he knew nothing could "give the like courage and confidence as the knowledge of God in Christ will." The superiority of these men--more intelligent than the common soldiers, better disciplined, better trained, better armed, excellent horsemen and fighting for a great cause--not only over the other parliamentary troops but over the royalists, was soon observed in battle. According to Clarendon the latter, though frequently victorious in a charge, could not rally afterwards, "whereas Cromwell's troops if they prevailed, or though they were beaten and routed, presently rallied again and stood in good order till they received new orders"; and the king's military successes dwindled in proportion to the gradual preponderance of Cromwell's troops in the parliamentary army. At first these picked men only existed in Cromwell's own troop, which, however, by frequent additions became the nucleus of a regiment, and by the time of the New Model included about 11,000 men.