In this article we intend to approach the study of the theories of imperialism and the specific characteristics of Marxism in early French Socialism, through the study of a particular author: Paul Louis (1872-1948), a militant of Blanquist origin that wrote a vast amount of work during his political life. Our first concerns are the specific features of his analysis of colonialism and imperialism: the economic foundations of the phenomenon, his characterization of the different imperialist countries and his political conclusions; tied to this, we analyze his position on some general debates of the socialism of that time: His standings on revisionism and socialists' ministerial participation, the general strike, the Russian Revolution of 1905, and the Republican form of government. This internal analysis of Paul Louis' work will be explained also in the framework of the debates over these topics that ran through French Socialism as a whole, as well as the political organizations in which he participated and the Second International, to be able to establish to which other positions they were close, and where were they placed in the fractional disputes on both national and international levels. Our hypothesis is that Paul Louis articulated an original vision of imperialism closely related to the political changes of his time, from some early analyses more related to a series of republican legacies, going through a rigorous analysis of imperialism as a phase of capitalism that influenced Lenin, until a final phase where he became a sort of party spokesman on the subject, returning to a more propagandistic stance. At the same time, from 1905 onwards he formed his own vision of Marxism that questioned many aspects of Guesdist orthodoxy, the predominant Marxist current of French Socialism.