Montrose was the unlikeliest Royalist hero. A Covenanter who had signed the National Covenant in 1638 against Charles I's religious policies, he had switched sides when he decided the Covenanter leadership had overreached. Sent to Scotland with a commission as Charles's Lieutenant General, he was at first ignored, then taken seriously, then feared. His army was a strange coalition: regiments of Irish Catholics under Alasdair MacColla sent over by the Confederate Catholics of Ireland, and varying numbers of Highland clansmen whose enthusiasm and presence fluctuated with the seasons and their feuds. With these men Montrose won at Tippermuir, Aberdeen, Inverlochy, Auldearn, Alford, and Kilsyth, a campaign as audacious as anything in seventeenth-century Europe. After Kilsyth the lowland towns lay open to him.

Then the cracks appeared. Montrose refused to let his men sack Glasgow, accepting £500 from the city instead, and then waived the money to spare the burgh the cost, leaving his army unpaid. The Highlanders under MacColla refused to march any further south, unwilling to leave Clan Campbell in their rear in the Highlands. Montrose appointed the Earl of Crawford as Lieutenant General of Horse, an act that affronted Lord Aboyne and the Gordon cavalry, who promptly went home. Montrose marched south to the Borders with barely 500 Irish musketeers and a small troop of horse, hoping the Border gentry would flock to him. Only a few did. Meanwhile, Sir David Leslie was riding back from England with the cavalry of the main Scottish army, 5,000 horse and 1,000 foot, intending to put a stop to Montrose.

Montrose quartered himself, his officers, and some of his cavalry in Selkirk on the night of 12 September. His infantry and the rest of the horse camped on flat ground across the Ettrick Water at Philiphaugh, behind ditches and hedges they had fortified with musketeers. The morning of 13 September was misty. Leslie, who had bivouacked at Melrose the night before and quietly driven in Montrose's outposts, advanced up the Tweed and divided his force into two wings. One attacked the Royalist position head-on, getting within half a mile before the alarm was raised. The other swung around in a flanking maneuver, probably along the south bank of the Ettrick. Montrose, hearing gunfire from his Selkirk lodging, raced to the battlefield to find his men in confusion. The Irish foot, behind their improvised defenses, repelled two Covenanter attacks. The flanking force broke them. Montrose led a hopeless charge with 100 horsemen against 2,000 dragoons, was urged by his friends that the Royalist cause in Scotland would die with him, and cut his way out with about 30 men toward Peebles.

About 100 men of Manus O'Cahan's Irish regiment kept fighting after the cavalry had fled. Eventually they surrendered on the promise of quarter, the standard protection given to prisoners of war. They were not given it. Presbyterian ministers travelling with Leslie's army persuaded him that mercy to Irish Catholics was foolish. The prisoners were killed in cold blood. Then the killing turned to the camp behind them: roughly 300 camp followers, many of them women and children who had come with the regiment from Ireland, were slaughtered. They were not soldiers. They had taken no part in the fighting. They had surrendered. They were murdered anyway because they were Irish and Catholic and the men who controlled their fate decided their lives did not count. The killing was carried out by a victorious army that called itself the godly side of the war. The ministers who urged it did so in the language of righteous purification. The victims left no monuments, and for centuries the massacre was treated as a footnote rather than the atrocity it was.

Montrose tried to raise another army in the Highlands and could not. He fought a guerilla campaign through the winter, then received orders from King Charles, by then himself a prisoner, to lay down his arms. He went into exile. He would return to Scotland one final time in 1650, be betrayed, captured, and hanged in Edinburgh. The likely site of the battlefield is now home to Selkirk Cricket Club and Selkirk Rugby Club, with a handful of cottages. Historic Scotland has inventoried and protected the site. A 2011 archaeological survey found only a handful of musket and pistol balls and a few seventeenth-century coins. The ballad collected by Francis James Child as number 202 remembers the battle. The 300 dead camp followers have no marker. Read the ballad, and the absence of their names becomes its own kind of monument.