Montrose had planned on taking an army from England to serve his cause in Scotland, and made his way to an audience with
Prince Rupert of The Rhine. Unfortunately, Montrose arrived just days after the English Royalist defeat at
Marston Moor on 2 July 1644. Rupert promptly commandeered most of Montrose's men to replace his own fallen numbers. Montrose decided to go to Scotland incognito, with two allies,
Sir William Rollo, and Sir James Sibbart. On the journey through his own country, Scotland, where he was now an outlaw, he learned of McColla's arrival, and raced to meet him. McColla and O'Cahan united with Montrose on the
Mull of Kintyre on 29 August 1644. The alliance proved to be formidable. They achieved major victories and won many smaller skirmishes through the heart of Scotland in what became known as their 'Year of Glory' (1644–1645). O'Cahan led a division on MacColla's behalf throughout the Montrose campaigns. The first victory came at
Tippermuir, on 1 September 1644. The next, at
Aberdeen, on 13 September 1644, was more controversial in that the Royalists, including O'Cahan's men, were involved in the massacre of hundreds of unarmed civilians throughout the city. Montrose wanted to expand his forces and march south, to England to help the King, who was by this time faring badly as
Cromwell's
New Model Army grew in strength. The bulk of the
Scottish Covenant army was now fighting for Parliament in England. However, the successes of Montrose and McColla made the Covenanters withdraw men from England to fight for their home soil. McColla, and the Scottish Highlanders who served with him alongside O'Cahan's Irishry had little interest in England, as they had their feud with the
Campbells to address. The Highland warriors who came to their aid frequently left the battlefields to carry home their spoils of war, so they often vanished for months on end, though most did return. The Scottish soldiers who served Montrose constantly drew him back from his planned advances on the English border to have another charge against the forces of the
Earl of Argyll, leader of the Campbell Clan. McColla was more loyal to Montrose, but he often had to separate from him to help recruit more men as the army's numbers waned. While McColla was away, O'Cahan usually stayed with Montrose, with whom he became a powerful ally. O'Cahan briefly traveled to Ulster to try to recruit more fighters, but was unsuccessful. When McColla was away on a recruitment drive on 21 October 1644, Montrose and O'Cahan and their men found themselves pinned down at
Fyvie Castle by Argyll's forces. O'Cahan led a daring night raid into the Campbell lines to break the siege. The
Campbells fled and O'Cahan grabbed the powder supplies abandoned by the deserters. He jokingly told Montrose "We must at them again, for the rogues have forgot to leave the bullets with the powder." It is one of the few direct quotations that history record from him. McColla returned soon after the battle at
Fyvie with a battle plan that Montrose regarded as impossible. McColla had raised a formidable body of
Scottish Highlanders, all of whom felt a desire to completely crush the Campbell Clan. They were predominantly
Macdonald allies like the
Clan Ranald. McColla proposed a raid through the heartland of Campbell-owned estates, in effect the complete destruction of
Argyllshire. The assault was to culminate in a near suicidal march on Argyll's personal estate at
Inverlochy, which was thought impossible to capture. The march on Inverlochy was made, despite Montrose's reservations, though he accompanied the force.
Argyllshire was indeed razed. The hundreds of square miles covered were more remarkable for the season–during the winter blizzards of early 1645, when the sea off the coast froze. The culminating attack, and massacre of Campbells at Inverlochy on 2 February 1645 was made after a two-day march over the foothills of
Ben Nevis. Argyll abandoned his men and sailed away on his personal galley to save himself. Inverlochy was MacColla and O'Cahan's greatest moment. It is recorded that O'Cahan personally drew first blood. Few Campbell captives were allowed to live. The year of glory was now past its peak. Montrose became increasingly overconfident and began to make tactical blunders. He wrote letters claiming that the victories were all his own doing, omitting the work of his Scots-Irish allies. == Decline and fall ==