Following a year of Royalist battlefield successes, in which they took Banbury, Oxford and Reading without conflict before storming Bristol, the Parliamentarians were left without an effective army in the west of England.

Essex reacted by making a surprise attack on the Royalist lines at dawn, capturing several pieces of high ground and leaving Charles on the back foot.

Although the numbers of casualties were relatively small (1,300 Royalists and 1,200 Parliamentarians), historians who have studied the battle consider it to be one of the most crucial of the First English Civil War, marking the high point of the Royalist advance and leading to the signing of the Solemn League and Covenant, which brought the Scottish Covenanters into the war on the side of Parliament and led to the eventual victory of the Parliamentarian cause.

After the inconclusive Battle of Edgehill in October, the Royalists advanced on London; after they were halted at the Turnham Green in November, Charles established his capital at Oxford.

Arguably the most comprehensive Royalist victory of the war, it isolated Parliament's garrisons in the west; reinforced by troops from Oxford under Prince Rupert, on 26 July they captured Bristol, gaining the second largest city in Britain.

[5] With the city captured, however, an immediate dispute occurred over who was to govern it, and this led to Charles travelling there on 1 August to take personal command of the Royalist forces.

[14] With the force assembled, Charles sent a group of heralds, escorted by 1,000 musketeers, forward at approximately 2:00 pm, at which point they read out the King's demands to a meeting of 26 local council and garrison officers, including Massie.

It was resolved that it was crucial that Gloucester was still to be taken; if it was left in Parliamentarian hands, it would act as a break in lines of communication should the Royalists advance further east towards London.

In addition, Charles's personal reputation had been sullied – travelling so far and yet not taking Gloucester would affect the respect and prestige accorded to him, about which he was "notoriously sensitive".

If they did attack, they would be exhausted and, according to Royalist intelligence, far weaker than the Oxford army, allowing Charles to destroy Parliament's one remaining significant force.

[24] Charles's cautious failure to directly assault the town, putting a higher priority on minimising losses than on victory, had cost the Royalists dearly; while claims for their number of dead and wounded men ranged from 1,000 to 1,500, only around 50 people inside the city were killed.

[26] The first alternative was to march southeast to the River Kennet and cross it, going through Newbury and returning to Reading's fortifications, thus evading the Royalists and allowing for a safe retreat to London.

These hardships meant that the Royalists arrived at Newbury before Essex, with both armies settling down for the night outside the town, too exhausted to immediately fight.

Even if Essex managed to cross the bridge, the other side of the river featured several hundred metres of waterlogged ground, which would slow his soldiers and leave them open to attack while necessitating the abandonment of the Parliamentarian artillery, a "major humiliation for a seventeenth-century army".

The Royalists were led by Charles I personally, with William Vavasour commanding the right wing, Prince Rupert the left, and Sir John Byron the centre.

[46] The cavalry were unable to make further gains, having engaged only a small part of the Royalist horse and being unwilling to press their attack against the larger body.

The Royalist council of war reconvened to discuss the events, and accounts suggest the meeting was acrimonious, with the fall of Round Hill described as "a most gross and absurd error".

Byron, in the meantime, was commanded to support an attack by the Royalist musketeers on Skippon's force, drawing his regiments up behind the infantry "ready to second them in case the enemy's horse should advance towards them".

Rupert's cavalry was too weak to defend against this advance due to its large firepower, and he instead ordered two regiments of foot commanded by John Belasyse to halt Essex.

The Parliamentarian records report they were "hotly charged by the enemies' horse and foot", who succeeded in forcing Essex slowly back, although the fight took four hours.

[58] Although the Royalists failed to press this attack due to the difficulty of manoeuvring cavalry in the field, and Essex briefly retook the ground, the loss of this infantry regiment opened a gap in the Parliamentarian line.

Although they succeeded in bridging the gap between the two wings of Essex's force, there was no cover, and a Royalist battery of eight heavy guns drawn up on high ground began firing on them.

Unable to move because of the necessity of their position, they were left enduring close-range fire "when men's bowels and brains flew in [their] faces", resisting two attacks by Royalist cavalry and infantry led by Jacob Astley.

Both army councils met; Essex's plan to force his way past the Royalists seemed feasible, and many Parliamentarians, loath to give up the ground they had taken, fully expected the battle to continue.

[64] The Parliamentarian force, now free of Charles's army, retreated towards Aldermaston as quickly as possible and eventually made it to Reading and then London, where Essex received a hero's welcome.

[70] John Barratt noted that the Royalists had failed in "what might prove to have been their best chance to destroy the principle field army of their opponents, and hopes of a crushing victory which would bring down the Parliamentarian 'war party' lay in ruins".

[71] The high Parliamentarian feelings after Newbury led to the signing of the Solemn League and Covenant, bringing a powerful Scottish army down to assault the Royalists.

"Thanks to the failure...to win a decisive victory there, the English, Welsh, Scottish and Irish subjects of all of King Charles' Three Kingdoms would henceforth play a bloody price in a steadily widening and deepening war".

[75][73] A more narrow view was taken by Sergeant Henry Foster, who fought with the London Trained Band in their attempt to prevent the Royalists splitting Essex's army.

The diary of Walter Yonge of Colyton also contains two reports written for the House of Commons by Essex's generals, including Stapleton, although the originals have been lost.