This chapter will examine Errico Malatesta’s (1853-1932) position on intervention in the First World War. This will include polemics within the Italian anarchist communities in Italy, London and the diaspora, and with the international anarchist movement, most notably with Kropotkin. The organising theme is the concept of the lesser evil in choosing war or peace. This chapter concludes with a discussion of the unintended consequences of the First World War, which posed challenges and crises for anarchism.