Strachan supported the anti-royalist faction and took command of the Scottish Parliamentary army which defeated Royalist general Montrose at the Battle of Carbisdale.
However the Scottish Parliament and a section of the Kirk party forged an alliance with Charles, Prince of Wales, offering him the crown of Scotland.
This faction demanded that the Act of Classes (1649) was enforced (removing Engagers from the army and other influential positions) and remonstrating against Charles being crowned King of Scotland.
[1][2] [3] For his perceived act of betrayal he was excommunicated by the Kirk in January 1651 and in April the same year declared a traitor suffering the forfeiture of his property.
Edward Furgol his biographer in the Oxford Dictionary of National Biography suggests that his relatively humble background, compared to most officers in Scotland at the time, who came from the landed classes, may have contributed to his radicalism which had more in common with the English independents in the New Model Army.
[5] He was prevented from remaining under English arms by the terms of the Self-denying Ordinance and joined the Scottish Army and by May 1645 had become major of Sir John Brown of Fordell's horse.
[7] He brought the news of Charles's execution to Edinburgh, and, after much discussion on account of the scandals of his past conduct, the commission of the Kirk on 14 March 1649 allowed him to sign the National Covenant.
Alexander Leslie, 1st Earl of Leven, wished to get rid of him as a "sectary", but the Kirk supported Strachan, and he for his part was eager to clear the army of malignants.
On 27 April he moved west, along the south side of the Kyle of Sutherland, near the head of which Montrose was encamped, in Carbisdale, with 1,200 foot (of which 450 men were Danes or Germans), but only forty horse.
After giving thanks to God on the field, the victors returned with their prisoners to Tain, and Strachan went south to receive his reward for winning the Battle of Carbisdale.
[9] Strachan was in such favour with the Kirk that they contributed one hundred thousand marks to raise a regiment for him, the best in the army which Leslie led against Cromwell.
[12] On 1 December troops from the Scottish Western Association army under Ker assaulted John Lambert's English forces in Hamilton but were beaten.