In January 1644, he accompanied the Scottish army into England as a member of the Committee of Both Kingdoms and in command of a troop of horse, but was soon compelled, in March, to return to suppress Royalists in the Scottish Civil War and to defend his own territories. He forced Huntly to retreat in April. In July, he advanced to abet the Irish troops now landed in Argyll, which were fighting in conjunction with Montrose, who had put himself at the head of the Royalist forces in Scotland. Neither general succeeded in obtaining an advantage over the other, or even in engaging in battle. Argyll then returned to Edinburgh, threw up his commission, and retired to Inveraray Castle. Montrose unexpectedly followed him in December, compelling him to flee to Roseneath, and devastating his territories. On 2 February 1645, while following Montrose northwards, Argyll was surprised by him at Inverlochy. He witnessed, from his barge on the lake to which he had retired after falling from his horse, a fearful slaughter of his troops, which included 1,500 of the Campbells. He arrived at Edinburgh on 12 February and was again present at Montrose's further great victory on 15 August at Kilsyth, whence he escaped to Newcastle. Argyll was at last delivered from his formidable antagonist by Montrose's final defeat at Philiphaugh on 12 September.