In January 1642 Charles I had abandoned London after it fell into Parliamentarian hands, and in October the two sides finally met in battle at Edgehill in Warwickshire. The outcome was not decisive, but the opportunity existed for the king to reach London before his enemies and reclaim it for the Royalists; it was squandered and the Royalist army was forced to winter in Oxford. Next spring they secured the West Country, seized control of the North and captured Bristol, but after being forced to abandon the siege of Gloucester Charles was defeated at Newbury on 20 September 1643 and balance was restored. In 1644 events in Scotland and Ireland came into play, with the Scots choosing to support Parliament and a peace of sorts in Ireland releasing forces to reinforce the Royalists.
At the end of June 1644 Charles I had the better of Sir William Waller's army at Cropredy Bridge, near Banbury, and the Royalists managed to evade the attempted pursuit. From Towcester the despondent Waller had written of Parliament's armies to the Committee of Both Kingdoms, which had overall command, saying:
My Lords, I write these particulars [of disaffection and desertion] to let you know an army compounded of these men will never go through with their service, and till you have an army merely your own that you may command it is in a manner impossible to do anything of importance.
In the North the Scots reinforced the Parliamentarian army, which defeated the king's forces at Marston Moor on 2 July, ending the Royalist control of the North, but Waller's view of affairs was reinforced by the failure to defeat and capture the king at the second battle of Newbury in October, to the discredit of the earl of Manchester; this event followed hard on the earl of Essex's defeat at Lostwithiel. The outcome was the creation of the New Model Army, and the events of 1645 would depend massively on the speed with which general officers could be replaced and newly formed units could become operationally effective.
Despite the satisfaction in Royalist circles at the escape from Newbury and the victory at Lostwithiel, the loss of the north of England was a serious setback. The hope of regaining the region, and the supplies and manpower that went with it, was slight given the continuing presence of the Scots army there. The queen was on the Continent attempting to raise reinforcements and supplies which would have to run the gauntlet of Parliament's navy. Further, the hope of raising men from South Wales had been frustrated by defeat at Montgomery. Sir William Brereton wrote the next day, 18 September 1644:
… the Lord so guided and encouraged our men, that with a fresh valiant charge we routed and put to retreat and flight their whole army … slew (I do believe) 500, wounded many more, took 1,500 prisoners… The enemy's army was reported (and I do believe it) no less than 4,000 — the foot being the old Irish, who came out of Ireland with Col. Broughton, Warren, Tillier…
Monmouth was taken a week later, but the Royalists retook it in November.
The Royalist army was quartered mainly west of Oxford that winter and no restructuring was undertaken other than a shift in commands. Prince Rupert became Lord General and his brother Maurice assumed command in Wales and the Marches. A number of minor actions were attempted, against Abingdon and against Weymouth; neither succeeded in changing the overall position. Parliament took the initiative when Sir William Brereton invested the important town of Chester and Maurice was ordered to relieve the place. To do so he took the remnants of the Irish regiments so badly cut up at Monmouth, now brigaded together as the 'Shrewsbury Foot', from that town, leaving it defended by a couple of hundred men. Brereton withdrew and Maurice entered Chester on 20 February, but Parliamentarian forces seized Shrewsbury in his absence and with it the supplies he had so painstakingly built up there. Moreover, the lines of communication in the West were now compromised.
The decisions taken by both sides in 1645 owed a great deal to the necessities of supporting their war machines. The South–East of England and East Anglia were controlled by Parliament. This gave them the major ports and centres of commerce and agriculture as well as, because of their possession of the navy, the control of much of the import and export business. Even more vital was the possession of the magazines of the Tower of London, the Arsenal at Woolwich and the stores at Greenwich and, outside the South–East, the military supplies at Hull and Portsmouth. To these resources was added the output of the arms manufactories in the South–East. Further, the making of gunpowder called for saltpetre (potassium nitrate), of which supplies in England were limited, and for sulphur, which had to be imported from abroad.
Also brought in from overseas were horses and armaments, so that control of the seaports was vital for bare survival, let alone victory. Parliament had not only London, but also King's Lynn, Hull and the south coast ports. This, combined with the loyalty of the navy, eased not only the challenge of obtaining materials, but also that of supplying forces elsewhere with men and arms.
The formation of the New Model Army early in 1645 brought with it a new system of procurement and supply for Parliamentarian troops. A central magazine was established at Reading and two new committees were set up to deal with procurement and distribution. The efficiency of this system contributed substantially to the creation of the New Model Army as an effective fighting force.
Charles I had his court and headquarters in Oxford and his lines of external supply depended on holding the port of Bristol in the South–West and Chester in the North)West to maintain contact with France and with Ireland. In Oxford new industry grew to meet the Royalist demand. As the war continued, Bristol, which had been taken in July 1643, became a major supplier to the Royalist armies, bringing in raw materials both from overseas and from Wales and the Forest of Dean. By February 1644 the Bristol gunsmiths were turning out 200 muskets and bandoliers a week. More industry grew in the West Midlands at Stourbridge and Dudley, sending its output to Oxford by boat down the Severn to Worcester and then overland by cart.
The importance of Ireland as an entrepôt for Royalist arms imports increased the reliance on the ports of the South–West in the supply chain of Charles's armies. The Irish confederates sent agents to Holland, Rome and the leading Catholic courts to acquire arms, while the Parliamentarians traded heavily through the east coast ports, mainly with Holland. Both sides were sensitive in the extreme to even a remote threat to these lines of supply. Carriage of supplies was conducted both by water and on land. Parliament used King's Lynn and London as distribution centres, moving goods to the magazine at Cambridge by the rivers Ouse and Cam and to the stores in Reading by way of the Thames. The supply of outposts such as Gloucester posed considerable problems. Convoys of wagons or pack–horses had to be protected by armed escorts drawn from the army. The Royalists' difficulties were even greater. The Thames served to move arms from Oxford to Abingdon and Wallingford, but most of the shipments were by road and were therefore vulnerable.
It was also necessary to provide troops with food and drink. As long as funds could be raised by various forms of taxation, payment for the basic requirements of bread, cheese and beer for the men and hay, oats and pulses for the horses could be made. Such payment was at times in the form of vouchers to be redeemed subsequently, and those of the Royalists, who were forced to increase their voucher use as the war continued and their taxable territory decreased, ultimately were not reimbursed. While an infantryman carried about one week's food supply in his snapsack, meat was obtained locally by purchase or plunder. The movement of troops around the country was accompanied by loss to the local population, sometimes compensated for, but loss all the same. This contributed to the emergence of irregular forces, armed with whatever they could lay their hands on, with a non–partisan animosity to each and every fighting force. The Clubmen, as they were known, were an additional source of annoyance to troops on the march.