The summer of 1643 the war was progressing very successfully for the royalists. The success had been underpinned by Hopton's brilliant campaign, moving out of Cornwall with victories at Braddock Down and Stratton and then driving Waller back eastward at Lansdown. In June, following the royalist victory over Waller at Roundway Down, Prince Rupert had captured the country's second wealthiest city, the key port of Bristol. By August most of Dorset and Devon, indeed most of the South West, was also in royalist control.

In the all important Severn valley, which was becoming the heart of the royalist war effort, only the strategically important city of Gloucester held out. It controlled both the major road and river communication in the region, standing between the royalist headquarters at Oxford and their suppliers, supporters and recruits in the West and in South Wales. Charles needed to secure the city. But despite few troops and poor provisions the parliamentarian Governor of Gloucester, Colonel Massey, refused to surrender. Unwilling to repeat the heavy losses that had resulted from the storming of Bristol, Charles besieged the city.

The fate of Gloucester was seen as a key factor likely to determine the outcome of the war. The parliamentarians in response were busily equipping and raising troops for Essex’s army in London. At the end of August  the Earl of Essex, with an army some 15,000 strong, set out from London the relief of Gloucester. On learning of Essex’s approach Charles raised the siege on the 5th September and withdrew southward. Gloucester had been relieved and re-supplied, but Essex still had to safely return his army to London. The royalists dogged Essex's route eastwards, with Rupert’s cavalry harrying their line and slowing their progress. It was Charles intention to bring Essex to battle on his own terms and, by destroying the principal parliamentary army, bring a swift end to the war.

Newbury was a key location on the main route back to London and the royalists managed to reach the town before Essex, effectively blocking his path to London. Battle was now inevitable. Essex had no choice but to force his way past to the capital. But in the Gloucester campaign Essex showed the best of his military abilities, and nowhere more so than in the action at Newbury.