For Leon Trotsky World War One was important because as it progressed he moved closer to Lenin. He joined the Bolsheviks in 1917 as a natural conclusion to a long-term process. He was, though, obscuring the truth, for as this work shows, between 1914 and 1917 the differences separating Trotsky from Lenin and the Bolsheviks actually increased. In this first comprehensive examination of Trotsky's thoughts on World War One, Thatcher upsets not only Trotsky's version of this crucial period in his life but also the standard account of Isaac Deutscher. He shows, for example, that Trotsky had much greater freedom of expression both in France and in Russia than claimed by Trotsky and his biographer. Based upon an extensive reading of primary and secondary sources, Thatcher shows clearly what issues concerned Trotsky most and when. Trotsky's thought is examined sympathetically as well as critically. Trotsky's wartime journalism is shown to be of continuing relevance to contemporary issues ranging from European unity to ethnic cleansing in the Balkans.