Surrey, Sussex and Hampshire). He fought bravely during Waller's unsuccessful assaults on Basing House during November 1643. Strachan remained with Waller as part of the South-Eastern Association until April 1644, when he became a captain and later major of horse in the Plymouth garrison. In October that year he appeared before the Committee of Both Kingdoms before taking command of a regiment of horse (cavalry). 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. By May 1645 he had become major in Sir John Brown's new regiment of horse. He participated in the destruction of the royalist Northern horse by Brown's regiment at the Battle of Annan Moor (20 and 21 October 1645). He continued in Scottish service until early February 1647. In 1648 he had rejoined the English Army and served under Cromwell at the Battle of Preston (1648), with the rank of major. According to Robert Baillie, his former life had been "very lewd", but he had reformed, "inclined much in opinion towards the sectaries", and remained with Cromwell till the death of Charles I. He was employed in the negotiations between the Duke of Argyll and Cromwell in September 1648.<ref> cites: Carlyle, Letter 75.</ref> 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. Strachan was given a troop of horse (cavalry), and helped to disperse the levies of Mackenzie of Pluscardine at Balveny on 8 May 1649. The levies numbered 1,200, but they were routed by 120 horsemen. 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. As to any danger from Montrose, he says, "If James Grahame land neir this quarters [Inverness], he will suddenly be de . . ed. And ther shalbe no need of the levy of knavis to the work tho they should be willing". When Montrose did land, in April 1650, Strachan made good his words. By Leslie's orders he advanced with two troops to Tain, and was there joined by three other troops,