The Ironsides were troopers in the Parliamentarian cavalry formed by English political leader Oliver Cromwell in the 17th century, during the English Civil War. The name came from "Old Ironsides", one of Cromwell's nicknames.
Cromwell first mustered a troop of cavalry (then referred to as "horse") at Huntingdon in Huntingdonshire, on 29 August 1642, early in the Civil War. John Desborough was quartermaster. The troop was late in being organised, and arrived too late to participate in the Battle of Edgehill, the first pitched battle of the war. Cromwell however did witness the defeat of the Parliamentarian horse at the battle and wrote to fellow Parliamentarian leader John Hampden,
Your troopers are most of them old decayed servingmen and tapsters; and their [the Royalists'] troopers are gentlemen's sons, younger sons and persons of quality; do you think that the spirits of such base and mean fellows [as ours] will ever be able to encounter gentlemen that have honour and courage and resolution in them?
It is evident that Cromwell's answer to his own question lay in religious conviction. Early in 1643, he was given a commission as Colonel and expanded his troop into a full regiment in the newly formed Army of the Eastern Association, under the command of Lord Grey of Warke and then the Earl of Manchester. By 11 September that year, he referred to them in a letter to his cousin Oliver St. John as a "lovely company". A champion of the "godly", Cromwell became notorious for appointing men of comparatively humble origins but stoutly-held Puritan beliefs as officers, who would then attract men of similar background and leanings to the regiment. He wrote to the Earl of Manchester, who disagreed with this policy,
I had rather have a plain russet-coated captain that knows what he fights for and loves what he knows, than that which you call a gentleman and is nothing else. I honour a gentleman that is so indeed.