Was the British left's support for the anti-Franco cause 'the most outstanding example of international solidarity in British history'? Here Lewis Mates considers this claim and argues that support for the anti-Franco cause was varied and multi-faceted. He analyses the 'Aid Spain movement': activities undertaken at grassroots level in support of the Spanish Republic. He explores the nature of grassroots support, its extent and depth, the motivations of activists, the institutions they operated through, and importantly, the role and impact of ideas on activism. Those within the British Left who did not embrace the Republic's cause are also examined as are the consequences of these divisions for the labour movement at its different levels from grassroots to national.Mates provides new perspectives on an important period of twentieth-century British history, contributing to debates about the nature of the British left, grassroots activism and popular political engagement in a contradictory epoch.