In August 1914, as war broke out, socialist parties across Europe offered support to their own governments. The Socialist International was shattered. This rush to defencism has traditionally been seen as a volte face in which the International's frequent protestations in favour of peace and international working-class solidarity were suddenly abandoned. The collapse has been variously ascribed to socialist helplessness, betrayal, or ideological incoherence. This article examines the International's attitudes to war and peace as developed and espoused in the decades before 1914, and finds that the decisions of the constituent socialist parties in 1914 were understandable within this context. Socialists were not abandoning past ideals, but attempting to put them into practice. The circumstances of modern war, however, made traditional distinctions – between aggressor and defensive belligerents, and between ‘progressive’ and ‘reactionary’ nations – difficult to maintain. For some socialists, this meant that socialists of every country had a certain justification in rallying to their nation's defence. For Lenin and the Bolsheviks, however, if no capitalist country could be considered innocent, then all must be guilty.