Leon Trotsky argued for a United States of Europe in "The Peace Programme" of 1915. Excerpts:
The economic unification of Europe, which offers colossal advantages to producer and consumer alike, in general to the whole cultural development, becomes the revolutionary task of the European proletariat in its struggle against imperialist protectionism and its instrument militarism.
The United States of Europe - without monarchies, standing armies and secret diplomacy - is therefore the most important integral part of the proletarian peace programme.
The ideologists and politicians of German imperialism frequently came forward, especially at the beginning of the war, with their programme of a European or at least a Central European "United States".
Certain opponents of the programme of the United States of Europe have used precisely this perspective as an argument that this idea can, under certain conditions, acquire a "reactionary" monarchist-imperialist content.
Yet it is precisely this perspective that provides the most graphic testimony in favour of the revolutionary viability of the slogan of the United States of Europe. Let us for a moment grant that German militarism succeeds in actually carrying out the compulsory half-union of Europe, just as Prussian militarism once achieved the half-union of Germany, what then would be the central slogan of the European proletariat?
Would it be the dissolution of the forced European coalition and the return of all people under the roof of isolated national states? Or the restoration of "autonomous" tariffs, "national" currencies, "national" social legislations and so forth? Certainly not.
The programme of the European revolutionary movement would then be: the destruction of the compulsory, anti-democratic form of the coalition with the preservation and furtherance of its foundations, in the form of complete annihilation of tariff barriers, the unification of legislation, above all of labour laws, etc. In other words, the slogan of the United States of Europe - without monarchies and standing armies - would under the indicated circumstances become the unifying and guiding slogan of the European revolution.
Let us assume the second possibility, namely, an "undecided" issue of the war. At the very beginning of the war the well-know professor Liszt, an advocate of "United Europe", argued that should the Germans fail to conquer their opponents the European unification would nevertheless be accomplished.
Even a partial overcoming of the obstacles would mean the establishment of an imperialist trust of European states, a predatory share-holding association.
And this perspective is on occasion adduced unjustifiably as proof of the "danger" of the slogan of the United States of Europe, whereas in reality this is the most graphic proof of its realistic and revolutionary significance.
If the capitalist states of Europe succeeded in merging into an imperialist trust, this would be a step forward as compared with the existing situations, or it would first of all create a unified, all-European material base for the working class movement. The proletariat would in this case have to fight not for the return to "autonomous" national states, but for the conversion of the imperialist state trust into a European Republican Federation.