In the morning of 15 August 1645, the armies marched towards each other, the Covenanters under Lieutenant-General William Baillie from the north-east and Montrose's Royalist army from their overnight billet in Kilsyth to the west. The Covenanter army probably numbered some 3,500 foot compared to about 3,000 Royalists under Montrose and Alasdair Mac Colla. However, Montrose probably had an advantage in cavalry, with perhaps 600 to Baillie's 300.
Wary of the enemy after his defeat at Alford in July, Baillie drew up his army in a strong defensive position while the Royalist army was still on the march. He seems to have been on the southern side of the Kelvin valley on Girnal Hill near Kelvinhead and Ruchill. He was reluctant to engage Montrose and the previous day had deliberately not come close enough to Montrose to risk coming to battle. However, he was under the direction of the Committee of Estates, who represented the theocracy that ran Covenanter Scotland. They had no intention of giving Montrose an opportunity to escape and ordered an advance towards Kilsyth. Then, unsatisfied with Baillie's initial defensive deployment, they directed Baillie to run his deployment to an adjacent hill on their right hand side (i.e. to the north), but this placed his forces at a great disadvantage as they could not make the move in battle formation. As Baillie attempted this difficult move, and before either army was fully deployed, they were drawn into action by the precipitate engagement of vanguard commanders on both sides. A fire-fight developed for control of cottages and enclosed gardens as Montrose's vanguard advanced in loose order up the glen. As the fight developed with an ill-disciplined counter-attack against the Covenanter vanguard, Montrose was forced to commit more troops as both the infantry and his supporting cavalry came under severe threat. With Baillie still desperately trying to deploy from line of march, his vanguard was driven back and the whole army collapsed into rout while Baillie himself tried unsuccessfully to bring in his reserves to hold the line. The whole Royalist army, horse and foot, now pursued the enemy, the pursuit and execution extending for some 14 miles. Baillie himself nearly came to grief in the rout as he became mired in Dullatur Bog; he was eventually able to escape to Stirling Castle, although few of his cavalry escort were so lucky.
Baillie, presumably aware of the advance of the Clydesdale forces, wished simply to shadow Montrose but not to engage. However, the Committee to which he had been subject since his defeat at Alford decided they should march to engage Montrose (Baillie 1775). When Montrose viewed the enemy in the morning and saw them advancing towards him, he marched out and met them.
Because the road was a 'rough and uneasy ' way' the Covenanter army marched nearby it, 'through the corns and over the braes' until they reached 'unpassable ground'. There Baillie embattled his army. Baillie claimed that nowhere in this location could a force greater than 'twenty men on front ' have gone from us or attacked us.' However, while Baillie did not wish to fight, the Committee did and they requested that the army move to the hill on their right hand. Baillie considered this poor ground, and ground that the enemy might gain before they did; also that in the move they would be at a disadvantage if engaged and even if the hill was taken it would be of no great advantage. His advice was in vain; the Committee visited the ground and most of the officers agreed to the move (Baillie 1775).
Meanwhile, probably to gain advantage of ground, Montrose sent a detachment forward 'to some houses where hee was to draw up' (anon 1645), described by Wishart as the commanding points on the field. These 'cottages and country gardens' were a vantage point which Montrose occupied with a 'small guard' of 100 men under Maclean of Treshnish, who were within sight of the main body of the Macleans and also of the Clanranald forces, their rivals even though in the same army (anon 1645).
It would appear that, as this was happening, the Covenanter army began to move from one hill to the other, against Baillie's better judgement. Most importantly, he says that the ground would not enable him to make this move with his forces in battle array, and it resulted in him in breaking the array and this placed his troops in great danger, even though he tried to shelter them from enemy view behind the brae. The commanded men with the horse marched to the fore, the foot regiments facing to the right and advanced to the hill, guided by Major Halden into an enclosure. But, without order from Baillie, who wished foot and horse to keep together, five files ('ratt') of musketeers advanced a musket shot to the fore of their main body. Baillie then rode over the brae to see the enemy posture, they being deployed in the meadow, with various commanded men 'falling up the glen through the bushes'. The latter presumably were the detachment sent forward by Montrose to take the houses and gardens.
On his return to the brae-head, Baillie found Major Halden, without orders, leading a party of musketeers over the field towards a house near the glen, and he would not retire when so ordered. Baillie then ordered Lauderdale's foot regiment, together with Balcarres' horse on their right, to march to the foot of the hill and turn to the fore, and Hume's foot to follow them and draw up to the side of him with the same front. Argyle's foot were then to draw up to the left of Hume's. But, without orders, Hume's advanced westward in amongst the dykes and the enemy, and Argyle's followed together with another body of horse. They took an enclosure from which it was impossible to withdraw, because the enemy was so close (Baillie 1775).
It must have been at this stage that the Covenanters began to fire on Montrose's commanded men. While this first action seems to have begun with Covenanter officers ignoring their commander, who did not yet have his army in battle array, the events that followed saw similar ill-discipline in Montrose's army, for at this time he had only two regiments placed, the horse not yet all come up and the whole of the rest of the foot was still at a great distance (anon 1645). In a complex broken terrain within which a few scattered houses in their enclosures provided potential strong points, a landscape most suited to skirmishing than to action in full battle array, the command structure in both armies seems to have disintegrated as individual commanders took rash decisions bringing the two armies into action before either commander was ready to engage. Though typical of the Highland and Irish forces throughout Montrose's campaign, it was less to be expected from the more disciplined Covenanter forces.
From Royalist accounts it seems the detachment sent forward by Montrose took the enclosures and then were attacked, but that they beat off the attackers who ran back in disorder (anon 1645), whereas Baillie's report suggest they advanced under fire to take the enclosures. Whatever the exact story, the events now evolved rapidly. Seeing the commanded men driving off the Covenanter musketeers, the Macleans and Clanranald regiments who numbered about 1,000 (Wishart 1720), without waiting for orders, marched
'along to the top of the bray, hard joining to the enemies whole body, both of horse and foot' (anon, 1645).
With this 'reckless charge' to the top of the hill they 'fell headlong on the entire army of the enemy', facing 2000 Covenanter foot and three troops of horse (Wishart 1720). This seems to be the stage where Baillie reports the Royalist foot reaching the next dike, fired upon at a distance by the Covenanter musketeers but at too long a range to be effective, then the rebels leaping the dike and breaking the regiments (Baillie 1775). It would have been impossible to withdraw them and yet they could not have withstood an enemy counter-attack.
However, the Covenanter rear was slow to advance and the van halted until they closed up (Wishart 1720). This seems to be the point at which Baillie was attempting to draw up all his forces in battle array. The collapse of discipline in the Covenanter army may have been individual commanders responding to the developing situation in their sector and attempting to counter the Royalist advance. It may be this phase of the action that Gordon reports when he says the Covenanters sent forward a vanguard of 3 regiments, one redcoats, within the middle of them two troops of horse, and one of lancers to flank them, to engage Montrose (Gordon & Dunn, 1844). The outcome was to leave Baillie desperately attempting to form up the remaining regiments which were still to hand, trying to bring in Crawford's regiment to fill the gap left by Hume and to bring up the Fife regiments as a reserve. Montrose saw Baillie's problem and, seeing the enemy 'beginning to stagger; and not all of them either drawn up', he commanded the vanguard with horse flanked by musketeers to charge, telling the rest to follow as they could (anon 1645). In a few words, Wishart glosses over the cavalry action that now followed. He says that Airlie's troop of horse responded and drove the Covenanter horse back upon their foot, driving the whole into confusion. The cavalry fled and the foot, 'after a faint resistance', firing a first salvo (anon 1645), threw down their arms and fled (Wishart 1720).
Gordon provides much more detail for this single episode: He says that the Covenanter horse advanced before the foot to a high ground but were 'sharply encountered' by Nathaniel Gordon's horse, who had been sent out in front of the royal army to gain the same high ground. Gordon beat them back within their foot, but carried his charge too far and was in danger, because the Royalist army was not fully deployed in battle array. The Covenanter foot let Gordon's cavalry in and then nearly encompassed them. M'Aleine and Glengarrie with their regiments had taken a higher ground and so stood too far off to give aid to Gordon, thus it was left to Aboyne to charge with just twelve horse upon the lancers flanking the red regiment, forcing them to retire behind the foot. He then turned to attack the foot, but as the pikemen in the front then responded he was forced to turn a little to the left and charge the flank of the regiment. There, the musketeers gave three volleys from the first three ranks, but he charged through them to relieve Gordon. Aboyne again charged the foot, who were attempting to retire in good order, breaking them and putting them to flight, while Airley, seconded by Colonel Gordon, broke the other two foot regiments, who threw down their arms and fled. The Royalist horse thus beat back the three Covenanter infantry regiments and their horse onto the main body of the Covenanter army, which had not yet been brought into order, causing them also to break and run. (Gordon & Dunn, 1844).
Even at this late stage of the battle, as his army disintegrated, Baillie claims he attempted to retrieve the situation, riding through the enclosures to find the reserve, but he found that they were already in flight. At the brook that they had crossed not long before, he attempted together with some of the Fife officers to make the Fife regiments stand to hold that pass, but they would not. Only then did Baillie himself flee the field (Baillie 1775).
The whole royal army now pursued the enemy and captured their cannon, with the Royalist foot, most of whom had played little part in the victory, taking the lead in the execution (Gordon & Dunn, 1844), a pursuit that extended for some 14 miles (anon 1645). It is said that no more than 100 foot escaped, and many of the cavalry were also killed, many others taken and the rest scattered in every direction (Wishart 1720). Though Reid argues that more infantry survived the pursuit than the contemporary accounts claim, it cannot be denied that with the pursuit at Kilsyth, the last substantial Covenanter army in Scotland was effectively destroyed.