In the historical battle, things went even worse, even though the Cornish Foot stayed in Devizes until the very end and took no part in the more crucial stage of the battle. The Royalists therefore, had only Horse and two light cannon (drakes, lighter than sakers). The battle was fought in very open fields, which clearly favoured the Royalist Horse. It was a disastrous defeat for Parliament. How could William Waller have made such a bad mistake after his successful defense at Lansdown?
To begin with, the Cornish Foot, all the army that the Royalists had West of Oxford, had been barricaded in Devizes, with their general Ralph Hopton badly wounded. If they surrendered, William Waller would have achieved a stupendous victory that could have decided the war. So understandably, he was willing to take his chances. The Cornish Foot were greater in number (about 3000-3500 compared to Waller's maybe 1500 ínfantry plus about 1000 or so Horse) but they were demoralised after the loss of Bevil Grenville and with Ralph Hopton wounded. They had begun negotiations for their surrender, whether with sincere intentions or not. Considering that they outnumbered Waller's Foote at least 2:1 and that Waller's Horse was useless in the streets of Devizes, which had been barricaded, and even more useless against the castle where Hopton had retreated, Waller understandably was prepared to accept a delay in the hope the Royalists might surrender.
The first Royalist Regiment of Horse that appeared from Oxford bringing much needed supplies was ambushed and routed. Prospects looked good and the best explanation for what followed was that William Waller did not expect that such a strong force of Royalist Horse would turn up just the day after the talks had started. The Kings Leutenant of Horse, Wilmot, had put together a scratch force of all the cavalry he could get, about 2000 strong and had ridden to relieve Hopton. William Waller, upon learning that such a cavalry force was heading his way that day, marched to intercept them before the Cornish should take courage and, even worse, sally out and attempt to unite with the Oxford Horse.
Presumably in moving on to interept them before they came close to Devizes, Waller could not find any ground that offered a good defensive position. Not only that, one has to assume there was no time to prepare man-made defenses. In fact he probably would have felt obliged to draw the Royalists to battle, because if they just rode off and united with the Cornish Foote they would have become an impossibly strong force to defeat. The prevailing view is that Waller actually deployed on a high hill, Roundway Hill, some 700 foot high, but still on relatively open ground, no tree cover, no hedges or walls or any other cover for the infantry.
What is less easy to explain is the behaviour of the Parliament Horse. Hasselrig's regiment was on the left of the Foot and Waller's with at least another regiment (presumably Popham's as he was present in the battle) was on the right of the Foot. Most estimates are that there were 6 Parliamentarian Regiments of Horse of up to 2000 men in total, but which regiments were these? Only five are mentioned in the summer battles: Waller's, Haselrig's, Popham's, Thomas Essex's and Burghill's and they had born most of the weight of the battle at Lansdown for Parliament. Richard Atkyns, who fought in the battle, talks only of Haselrig's regiment on the right wing. Each cavalry wing would have faced superior numbers. They may have been outnumbered by as much as 2:1 or more, especially Haselrig's Horse that had been so heavily involved in Lansdown.
I have no proposals of what one could have done under the circumstances but Willliam Waller seemed to have set himself up for defeat which was what happened. One advantage the Parliament Horse had was that they were more rested than the Royalist Horse that had marched from Oxford. Whether there were 3 regiments or 6 regiments, they were probably of good quality, having had recent battle experience. Despite being outnumbered, they could have drawn out their front and charged full speed the Oxford Horse. The worst that would have happened was they would have been defeated but, if so, they would have had a better chance to escape.
What actually happened was that they stood where they were, perhaps hoping to frighten the Royalists and deter them from battle, or who knows what. Worse still, they formed a small front several ranks deep, whereas Popham's (presumably) regiment had formed not to the side but behind Waller's Regiment of Horse. So on both wings the front of the Parliament Horse was very narrow. This would have been ok for a charge on infantry but the Royalists had no infantry, apparently not even dragoons. The Parliament Horse did not charge anyway, they just stood there and allowed themselves to be charged on both sides by the much more numerous Royalist Horse. Horses are herd animals and no matter what their riders intend, they have an instinct to follow the direction of movement of other galloping horses. So whenever Horse charged standing Horse, the defenders were usually routed because the overall vector of movement induced by that charge is unfavourable to the defenders. Isaac Newton did not live then and Newtonian physics had not been discovered, neither people understood much about biology and animal behaviour, but in practice whenever there were horse battles the one who charged first generally routed the defender, so that ought to tell you something.
The Parliament Horse, finding itself surrounded by the greater numbers of Royalist horsemen, was put to rout on both sides without much of a fight. The situation was worst on the right wing. The Parliament Horse fled ahead of the Royalist Horse, unwittingly in the direction of a steep cliff. If you have 1500 horses galloping behind you, you would not even think of stopping, so the herd behaviour of the horses led that entire wing of Parliament Horse down the pecipice, many of them being killed. A few appeared to have escaped to the rear of the Parliament Foot, among them Alexander Popham, William Waller and presumably also John Locke, as he survived the war, though it is not known if he was at the battle. But the majority of the Parliament Horse had been routed. Some 1000-1500 Foote were now facing 2000 Horse plus about 3000-3500 Royalist Foote that came out of Devizes to participate in the battle. The odds were now hopeless. The Parliament army was completely destroyed and it was even lucky that all the generals survived, without even one of them being taken prisoner. Actually, Arthur Haselrig's Horse had been wounded and he was about to be taken prisoner when some of his men came to his aid and he managed somehow to escape in the confusion.
William Waller had gambled everything and had lost. Instead of a great triumph, there had been a crushing defeat. Now the Southwest of England was practically in the hands of the Royalists except for a few fortified towns. Bristol was taken by the Royalists, who now had at last a big city and harbour in the South of England. This was regarded as the high point of the English Civil War for the King.

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[MT2W FKoC AAR] Times full of Distemper
Reviewed by
robinzx at the
Critic's Quill, Issue 31
Medieval 2 Total War “For King or Country” mod:
http://www.forkingorcountry.com/