If twentieth century politics in Wales has largely been defined by class, and therefore along the typical cleavage of Labour versus Conservative; it is nevertheless true that for a significant proportion of Welsh activists and voters, the cleavage is between nation and union (identifiable with the British state). Closely identified with Plaid Cymru, the Welsh nationalist party, a political manifestation of the Welsh nation was a direct inheritance from nineteenth-century liberalism and its persistence for much of the postwar period was a result of the persistence of that form of politics. But there was an alternative form of left nationalism that emerged through the Communist Party of Great Britain, which this chapter focuses its attention on. Beginning in the 1930s, and spanning almost the entire life of the party thereafter, communists engaged with and developed ideas about nationalism, nationhood and national liberation. This chapter considers the development of these ideas and argues that rather than Plaid Cymru, it was the Communist Party of Great Britain that enabled the persistence of left-nationalist thought and action after 1945 and that it was, to a large extent, communist activists who were the most consistently nationalist in that period.