After the Manifesto appeared, revolutions occurred in France and Germany, and the Belgian government, fearful that the revolutionary tide would engulf Belgium, banished Marx. He thereupon went first to Paris and then to the Rhineland. In Cologne he established and edited a communist periodical, the Neue Rheinische Zeitung, and engaged in organizing activities. In 1849 Marx was arrested and tried in Cologne on a charge of incitement to armed insurrection; he was acquitted but was expelled from Germany, and the Neue Rheinische Zeitung was suppressed. Later in the same year he was again banished from France and spent the remainder of his life in London.

In England Marx devoted himself to study and writing and to efforts to build an international communist movement. During this period he wrote a number of works that are regarded as classics of communist theory. These include his greatest work, Das Kapital (vol. 1, 1867; vols. 2 and 3, edited by Engels and pub. posthumously in 1885 and 1894, respectively; trans. 1907-1909), a systematic and historical analysis of the economy of the capitalist system of society, in which he developed the theory of the exploitation of the working class by capitalists through the appropriation by the latter of the "surplus value" produced by the former. See Capital.

Marx's next work, The Civil War in France (1871), analysed the experience of the short-lived revolutionary government established in Paris during the Franco-German War, known as the Paris Commune. In this work Marx interpreted the formation and existence of the Commune as a historical confirmation of his theory of the necessity for workers to seize political power by armed insurrection and then to destroy the capitalist state; he hailed the Commune as "the finally discovered political form under which the economic emancipation of labour could take place". This theory was explicitly projected in The Gotha Programme (1875; trans. 1922): "Between the capitalist and communist systems of society lies the period of the revolutionary transformation of the one into the other. This corresponds to a political transition period, whose state can be nothing else but the revolutionary dictatorship of the proletariat". During his residence in England Marx also contributed articles on contemporary political and social events to newspapers in Europe and the United States. He was a correspondent of the New York Tribune, edited by Horace Greeley, from 1851 to 1862, and in 1857 and 1858 he wrote a number of articles for the New American Cyclopedia, edited jointly by the American writer and editor Charles Anderson Dana and George Ripley.