The Second Civil War (1648) began in south Wales. The present study is the first detailed contextual narrative and analysis of the revolt in Wales, covering the originating mutiny at Pembroke Castle, its development into a rebellion that spread through south Wales, the Battle of St. Fagans (the biggest battle ever fought in Wales), and Cromwell's successful Welsh campaign involving the sieges of Chepstow, Tenby and Pembroke. The consequences of the revolt, involving the emergence of new local ruling elites and the cementing of a closer relationship with London central government, are examined.The study thus places the revolt in the context of events beyond the region, and is based on extensive original research of contemporary documents.The revolt has traditionally been seen as a Royalist uprising. This study offers a radical reinterpretation, viewing the episode instead as an expression of localist dissatisfaction at the post-First Civil War settlement, which was seen as disregarding local interests in favour of administrative and ideological centralisation focused on a concept of national government. The failure of the revolt thereby marked a subjugation of local by national interests – implemented by new ruling elites, and underpinned by a national army, the New Model – that fostered the closer integration of Wales into the Interregnal state.