The impact of the Sino-Japanese War on Britain has generally been overshadowed by the impact of the Spanish Civil War, which broke out a year earlier. Indeed, the only book on the subject, Arthur Clegg's Aid China (1989), is subtitled A Memoir of a Forgotten Campaign. Yet, for a few months in the autumn of 1937, these two campaigns achieved a kind of parity in British public perception. British opinion was united in condemnation of the Japanese bombing of Chinese cities, and, at its peak, the ensuing campaign attracted a broader range of supporters than the movement in solidarity with the Spanish Republic. For instance, the Archbishop of Canterbury publicly criticised Japan's actions in a way that would have been unthinkable in the case of Franco's Spain. Moreover, some acts of solidarity with China (such as the refusal by British dockers to load Japanese ships) went beyond what the supporters of the Spanish Republic could hope to achieve. This article makes a comparison of the two campaigns, and examines the interconnections between them. It not only sheds new light on the ‘forgotten’ campaign for China, but also asks why Spain – unlike China – became the ‘Great Cause’ of the later 1930s.