James Harrington’s Commonwealth of Oceana (1656) has been regarded as a republican challenge to Oliver Cromwell’s monarchial ambition. This interpretation, however, is hampered by its neglect of the historical context shaped by Cromwell’s conquests of Ireland and Scotland during the period of 1649 and 1652 and the subsequent inauguration of the Protectorate in 1653 to rule a United Commonwealth of Great Britain. It also fails to recognize Harrington’s ideological commitment to envisaging an English utopia at a time when England’s regal union with Scotland had been severed with the head of Charles Stuart in 1649. As an intellectual heir to Thomas More who had presented an ideal republic in his Utopia a century earlier, Harrington attempted in Oceana to show what a republic might look like in an archipelagic setting. Examination of this multi-layered historical milieu demonstrates that Harrington cared about Cromwell’s success by advising him of what to do in order to build the United Commonwealth not just of England but of the British Isles. Taking this British approach helps to spot what had been neglected in the scholarship on the republican debate of the 1650s: a Scottish problem. At mid-century England’s full-fledged incorporation of Scotland was yet to be completed. Reading Oceana with a new focus on this Anglo-Scottish dimension illumines Harrington’s intellectual project as an attempt ideologically to counter contemporary presbyterians’ efforts to reaffirm the Solemn League and Covenant adopted by the English Long Parliament and Scots Covenanters in 1643. Harrington grappled with how to remake Anglo-Scottish relations without recourse to the covenanting ideology seeking a confessional, confederal union of England and Scotland. His utopian commonwealth revealed how and why it would be possible for England to be a republic while having Scotland as its conquered province. This project involved enlarging the British electorate by emancipating the Scots peasantry from their feudal lords in search of an Anglo-Britannic empire within the archipelagic scope. If Utopia showed what it meant to write about a republic when it was not feasible, Oceana showed what it meant to write about a republic while living in an imperial polity.