This text offers an examination of Welsh military organization, strategy, tactics and conduct in war which goes beyond the military sphere and touches on all aspects of Welsh society in this period. It helps to explain the transformation of the country from a part of post-Roman medieval Europe to the series of independent principalities eventually conquered by Edward I. The author questions many of the traditional assumptions, notably the tendency to see the Welsh (and the Irish and the Scottish) as "barbaric" and provides a corrective to these current views. It demonstrates that, whilst allowing for differences given the nature, economy, geography and topography of the country, Welsh military developments from the end of Roman rule to the Edwardian conquest generally correspond to those seen in the rest of Western Europe.