The Colonial Ironsides is a comprehensive survey of the role played by Oliver Cromwell's expeditionary forces in subduing Royalist outposts abroad after the conclusion of the Civil Wars in the Three Kingdoms, the Channel Islands, the Isle of Man, the Scillies, the West Indies and North America. Beyond that, the book details the launch of the Western Design against Spain in the Caribbean and during its course, the failure of the expedition against San Domingo and the conquest of Jamaica. In Europe, the book studies the role of Cromwell's expeditionary brigade, and Charles II's army in exile, in the struggle between France and Spain in the Low Countries during the last years of the Protectorate: the Battle of the Dunes, the sieges of Dunkirk, Ypres, Oudenaarde and other towns leading to the acquisition of Dunkirk and Mardyke as a British colony on the Continent, the first since the loss of Calais under Mary Tudor.

The book is, in some ways, a prequel to the author's earlier book published by Helion & Co: The Last Ironsides: The English Expedition to Portugal 1662-1668. As such it shows how the troops that later went to Portugal and Tangier after Charles II's restoration in 1660 came to be at Dunkirk. Overall, the book rediscovers a series of episodes in British military history that have been forgotten, not detailed fully since Victorian times. No other modern books describe the full extent of this period of conflict, although there are books which detail several of the campaigns. This decade of early expeditionary warfare marked a change in English foreign policy from raiding the colonies of others, chiefly Spain, to becoming an imperial power.