Born at Musselburgh near Edinburgh to an obscure family, Archibald Strachan served in the army of Sir William Waller during the English Civil War. He was promoted to the rank of major after the battle of Lansdown in July 1643 and was praised by Waller for his courage during the attack on Basing House in November. By April 1644, Strachan was commander of Waller's regiment of dragoons. After further service in Parliament's armies in southern England, Strachan returned to Scotland until 1647. A radical in religion, he fiercely opposed the Engagement between Scotland and King Charles and served as a major in Cromwell's army on the Preston campaign of 1648. Strachan accompanied Cromwell into Scotland and participated in the negotiations with the Whiggamores that resulted in the Treaty of Stirling in September 1648.
In February 1649, Strachan's opposition to the Engagers secured him an appointment as lieutenant-colonel to Colonel Ker in the Army of the Covenant, though Lieutenant-General Leslie distrusted him because of his extreme religious views. Inspired by the biblical hero Gideon, Ker and Strachan believed that an élite force of godly warriors would always prevail over the hosts of the unrighteous. They routed the Royalist insurgents of Pluscardine's uprising at Balvenie in May 1649 and Strachan led Leslie's advance guard to win a decisive victory over the Marquis of Montrose at the battle of Carbisdale in April 1650. He was rewarded with £1,000 and a gold chain by the Scottish Parliament. The following July, Strachan was appointed colonel of a new cavalry regiment raised from funds contributed by the Scottish clergy.
Strachan was among those who distrusted King Charles II and doubted his sincerity in signing the Covenant. He fully supported the purges of the Scottish army ordered by the Kirk Party and was among the Covenanters who held secret talks with Cromwell and Lambert in the vain hope of finding a peaceful way to drive King Charles from Scotland. However, he fought courageously against the English at the battle of Dunbar in September 1650 where his regiment was on the Scottish right flank. He blamed Leslie for the Scottish defeat and refused to serve under him. The Committee of Estates sent Strachan and fellow radicals Colonel Ker and Sir John Chiesley to raise levies in south-western Scotland for the newly-revived Western Association army. The western army was characterised by strict adherence to the Covenant. In October 1650, its commanders issued a Remonstrance severely critical of King Charles and of all Scots who supported him. The Remonstrants pledged to remain neutral in the war against the English until the King genuinely repented and embraced the Covenant, but Strachan took the extreme view that the King should be arrested or exiled. On learning that Strachan was in communication with Cromwell, the Committee of Estates ordered his dismissal from his regiment.
The defeat of Colonel Ker and the Western Association army by Major-General Lambert at the battle of Hamilton in December 1650 ended all hope that the Remonstrants would prevail. Regarding King Charles and his supporters as a greater threat to the Covenant than Cromwell and the New Model Army, Strachan defected to the English with a small body of followers. The Kirk excommunicated him in January 1651 and he was denounced as a traitor in March. Strachan died in November 1652, reportedly in despair over his excommunication.
References:
Edward M. Furgol, Archibald Strachan, Oxford DNB, 2004
David Stevenson, Revolution & Counter-Revolution in Scotland 1644-51, 1977