Let us now examine how Lenin and Trotsky, the most important Marxist theoreticians living in the epoch of imperialism, viewed the issue of conflicts between the imperialist nation-states and compared them with the formation of imperialist federations.
A basic tenet for Marxists is their fundamental opposition to the imperialist state. In the epoch of monopoly capitalism, the imperialist nation-state – as well as any federation of states – has become reactionary. While the struggle for national independence from imperialist domination retains a progressive element in semi-colonial countries, this is not at all the case in imperialist states. This is why Lenin and the Bolsheviks denounced in 1914 all those “Marxists” who defended “their” imperialist fatherland by referring to the right of national self-determination. While such a defense of the fatherland is legitimate for a semi-colonial country, this is not the case for those countries which have already become powers which dominate other, smaller and more backward peoples.
Lenin emphasized that since the beginning of the epoch of imperialism, the defense of the fatherland in the advanced capitalist countries has lost any progressive element:
“What do we mean when we say that national states have become fetters, etc.? We have in mind the advanced capitalist countries, above all Germany, France, England, whose participation in the present war has been the chief factor in making it an imperialist war. In these countries, which hitherto have been in the van of mankind, particularly in 1789-1871, the process of forming national states has been consummated. In these countries the national movement is a thing of an irrevocable past, and it would be an absurd reactionary utopia to try to revive it. The national movement of the French, English, Germans has long been completed. In these countries history’s next step is a different one: liberated nations have become transformed into oppressor nations, into nations of imperialist rapine, nations that are going through the “eve of the collapse of capitalism”” 
Lenin warned against the illusion of transforming imperialist states into “non-imperialist” capitalist states. Likewise there could be no “league of equal nations under capitalism”.
„Finally, our “peace programme” must explain that the imperialist powers and the imperialist bourgeoisie cannot grant a democratic peace. Such a peace must be sought for and fought for, not in the past, not in a reactionary utopia of a non-imperialist capitalism, not in a league of equal nations under capitalism, but in the future, in the socialist revolution of the proletariat. Not a single fundamental democratic demand can be achieved to any considerable extent, or with any degree of permanency, in the advanced imperialist states, except through revolutionary battles under the banner of socialism.“ 
From this follows that socialists must not preach support for the imperialist nation-state. They must not lend any support to “their” imperialist fatherland in a conflict with another. This is the fundamental principle from which Lenin and the Bolsheviks derived their famous program of revolutionary defeatism. They consistently refused any form of support for the defense of the imperialist fatherland and stood for the defeat of their own ruling class. 
The core idea of Lenin’s approach was the struggle against the imperialist wars through the methods of the class struggle and the utilization of the crisis caused by the war for the revolutionary overthrow of one owns bourgeoisie. Hence the unequivocal stance for the defeat of one’s own government in the war:
„During a reactionary war a revolutionary class cannot but desire the defeat of its government. This is axiomatic, and disputed only by conscious partisans or helpless satellites of the social-chauvinists.“ 
This approach was combined with the struggle for the socialist revolution. Hence the central slogan of the Bolsheviks was the “civil war”:
„The conversion of the present imperialist war into a civil war is the only correct proletarian slogan,“ 
Based on the same method, when the League of Nations (the predecessor organization of the imperialist-dominated United Nations) imposed economic sanctions against fascist Italy (after the latter’s invasion of Ethiopia), Trotsky explained that the workers’ movement must not support such sanctions since they could only contribute to the inter-imperialist rivalry.
‘‘Most dangerous of all, however, is the Stalinist policy. The parties of the Communist International try to appeal especially to the more revolutionary workers by denouncing the League (a denunciation that is an apology), by asking for ‘workers’ sanctions,’ and then nevertheless saying: ‘We must use the League when it is for sanctions.’ They seek to hitch the revolutionary workers to the shafts so that they can draw the cart of the League. (…) The truth is that if the workers begin their own sanctions against Italy, their action inevitably strikes at their own capitalists, and the League would be compelled to drop all sanctions. It proposes them now just because the workers’ voices are muted in every country. Workers’ action can begin only by absolute opposition to the national bourgeoisie and its international combinations. Support of the League and support of workers’ actions are fire and water; they cannot be united.’’ 
In a major programmatic document, Trotsky emphasized that the workers’ movement will be prepared for the struggle against imperialist wars if it will only learn to oppose ”its” imperialist state times of peace.
“The defense of the national state, first of all in Balkanized Europe – the cradle of the national state – is in the full sense of the word a reactionary task. The national state with its borders, passports, monetary system, customs and the army for the protection of customs has become a frightful impediment to the economic and cultural development of humanity. The task of the proletariat is not the defense of the national state but its complete and final liquidation. (…) A “socialist” who preaches national defense is a petty-bourgeois reactionary at the service of decaying capitalism. Not to bind itself to the national state in time of war, to follow not the war map but the map of the class struggle, is possible only for that party that has already declared irreconcilable war on the national state in time of peace. Only by realizing fully the objectively reactionary role of the imperialist state can the proletarian vanguard become invulnerable to all types of social patriotism. This means that a real break with the ideology and policy of “national defense” is possible only from the standpoint of the international proletarian revolution.” 
In the struggle against the imperialist rivalry between the great European powers Trotsky advocated the slogan of the republican United States of Europe. There has been a process of discussion and elaboration among Marxist theoreticians about the applicability of this slogan with which we have dealt with elsewhere.  At this point we will limit ourselves to the following observations.
Lenin initially supported the slogan republican United States of Europe because it combined the struggle against the imperialist chauvinism and the limitations of the nation-state with a revolutionary-democratic program of fighting against the absolutist monarchies in Russia, Germany, and Austria. However, he later opposed the same slogan because, as he explained, it could either create the illusion of a peaceful and democratic unification of Europe under capitalism or it could become a slogan apologizing for the creation of an alliance of imperialist great powers.  This was the meaning of Lenin’s famous dictum that the slogan of the United States of Europe, under capitalist conditions, “is either impossible or reactionary.”
“Of course, temporary agreements are possible between capitalists and between states. In this sense a United States of Europe is possible as an agreement between the European capitalists ... but to what end? Only for the purpose of jointly suppressing socialism in Europe, of jointly protecting colonial booty against Japan and America, who have been badly done out of their share by the present partition of colonies, and the increase of whose might during the last fifty years has been immeasurably more rapid than that of backward and monarchist Europe, now turning senile. Compared with the United States of America, Europe as a whole denotes economic stagnation. On the present economic basis, i.e., under capitalism, a United States of Europe would signify an organisation of reaction to retard America’s more rapid development. The times when the cause of democracy and socialism was associated only with Europe alone have gone for ever.” 
Hence, Lenin concluded: “From the standpoint of the economic conditions of imperialism — i.e., the export of capital arid the division of the world by the “advanced” and “civilised” colonial powers — a United States of Europe, under capitalism, is either impossible or reactionary.” 
Lenin warned that the slogan of the United States of Europe could become a slogan which could be exploited by those social-imperialists who do not orientate towards a single nation-state but to the unification of European capitalism in order to strengthen Europe’s role as a dominant power in the world.
“Hobson, the social-liberal, fails to see that this “counteraction” can be offered only by the revolutionary proletariat and only in the form of a social revolution. But then he is a social-liberal! Nevertheless, as early as 1902 he had an excellent insight into the meaning and significance of a “United States of Europe” (be it said for the benefit of Trotsky the Kautskyite!) and of all that is now being glossed over by the hypocritical Kautskyites of various countries, namely, that the opportunists (socialchauvinists) are working hand in glove with the imperialist bourgeoisie precisely towards creating an imperialist Europe on the backs of Asia and Africa, and that objectively the opportunists are a section of the petty bourgeoisie and of certain strata of the working class who have been bribed out of imperialist superprofits and converted into watchdogs of capitalism and corrupters of the labour movement.” 
Taking Lenin’s criticism into account, Trotsky overcame the weakness of his initial version of the slogan of the United States of Europe by combining it with a clear class character. Hence, he reformulated the slogan in such a way that it combined the task of overcoming the “balkanization” of Europe into numerous nation-states by means of a federation the task of which would be the overthrow of capitalist class rule. Hence he argued in summer 1923 for the United Socialist States of Europe and won over the Communist International to this perspective.
“The sooner the popular masses of Europe regain the confidence in their own strength which was sapped by the war, and the more closely they rally around the slogan of “United Workers’ and Peasants’ Republics of Europe”, the more rapidly will the revolution develop on both sides of the Atlantic.” 
Unsurprisingly, the Stalinist bureaucracy dropped this slogan in 1928 since it (rightly) felt to be in contradiction to the national-centered and reformist perspective of “socialism in one country.” Against this, Trotsky unreservedly defended the internationalist program.
“But the Communist parties have their hands tied. The living slogan, with a profound historical content, has been expunged from the program of the Comintern solely in the interests of the struggle against the Opposition. All the more decisively must the Opposition raise this slogan. In the person of the Opposition the vanguard of the European proletariat tells its present rulers: In order to unify Europe it is first of all necessary to wrest power out of your hands. We will do it. We will unite Europe. We will unite it against the hostile capitalist world. We will turn it into a mighty drill-ground of militant socialism. We will make it the cornerstone of the World Socialist Federation.” 
He continued to raise the slogan of the United Socialist States of Europe in a number of major programmatic documents. In his last major document, the Manifesto of the Fourth International on the Imperialist War which was adopted by the Emergency Conference of the Fourth International in May 1940, Trotsky made clear that the imperialist nation-state is not an alternative but an historic regression. Likewise he denounced any imperialist unification of Europe as reactionary.
“The promise of the Allies to create a democratic European federation this time is the crudest of all pacifist lies. The state is not an abstraction but the instrument of monopoly capitalism. So long as trusts and banks are not expropriated for the benefit of the people, the struggle between states is just as inevitable as the struggle between the trusts themselves. Voluntary renunciation by the most powerful state of the advantage given by its strength is as ridiculous a utopia as voluntary division of capital funds among the trusts. So long as capitalist property is preserved, a democratic “federation” would be nothing but a worse repetition of the League of Nations, containing all its vices minus only its illusions. In vain do the imperialist masters of destiny attempt to revive a program of salvation which was completely discredited by the experience of the past decades. In vain do their petty bourgeois flunkies warm up pacifist panaceas which long ago changed into their own caricature. The advanced workers will not be duped. Peace will not be concluded by those forces now waging war. The workers and soldiers will dictate their own program of peace!” 
After we have outlined above the Marxist position on European unification, we will now analyze the positions of the British left on this issue and their tactical conclusions.
 V. I. Lenin: A Caricature of Marxism and Imperialist Economism; in: LCW Vol. 23, p.38
 V. I. Lenin: The Peace Programme (1916), in: LCW 22, pp. 167-168
 The RCIT has summarized its position on imperialist wars in its programme in the following way: “In imperialist wars, we reject any support for the ruling class. We advocate the defeat of the imperialist state. Our slogan is that of Karl Liebknecht: “The main enemy is at home”. Our goal is to transform the imperialist war into a civil war against the ruling class.” (RCIT: The Revolutionary Communist Manifesto, p.62) For a closer look on the elaboration of the Bolshevik concept of defeatism we refer to a study which the author of these lines has published some years ago in German language: Michael Pröbsting: Umwandlung des imperialistischen Krieges in den Bürgerkrieg. Die Strategie Lenins und der Bolschewiki; in: Revolutionärer Marxismus Nr. 40 (2009), pp. 58-94, http://www.thecommunists.net/theory/lenin-und-der-imperialistische-krieg
 V.I. Lenin: The Defeat of one’s own Government in the Imperialist War (1915); in: LCW 21, p.275
 V.I. Lenin: The War and Russian Social-Democracy (1914); in: LCW 21, p.34
 Leon Trotsky: Once Again the ILP (November 1935); in: Trotsky Writings 1935-36, p. 201
 Leon Trotsky: War and the Fourth International (1934); in: Trotsky Writings 1933-34, pp. 304-305 (Emphasize in the original)
 See Michael Pröbsting: Die Frage der Vereinigung Europas im Lichte der marxistischen Theorie. Zur Frage eines supranationalen Staatsapparates des EU-Imperialismus und der marxistischen Staatstheorie. Die Diskussion zur Losung der Vereinigten Sozialistischen Staaten von Europa bei Lenin und Trotzki und ihre Anwendung unter den heutigen Bedingungen des Klassenkampfes, in: Unter der Fahne der Revolution“ (FAREV) Nr. 2/3 (2008), http://www.thecommunists.net/theory/marxismus-und-eu/. See on this also Stephen Dabydeen: Trotsky, the United States of Europe and National Self-Determination, in: Hillel Ticktin and Michael Cox (Editors): The Ideas of Leon Trotsky, Porcupine Press, London 1995, pp.163-186.
 While the main motivation of Lenin’s refusal of this slogan were his studies on imperialism, he was also influenced on this issue by the criticism of Karl Radek, his close collaborator in the Zimmerwalder Left, and Rosa Luxemburg. See on this the report of a delegate at the Bern conference of the Bolshevik party in February 1915: G. L. Shklovsky: The United States of Europe Debate (1925) in; Lenin’s Struggle for a Revolutionary International. Documents 1907-1916. The Prepatory Years, New York 1986, pp. 251-252. See also R. Craig Nation: War on War. Lenin, the Zimmerwald Left, and the Origins of Communist Internationalism; Durham 1989, pp. 43-44.
79 W.I. Lenin: Über die Losung der Vereinigten
 V. I. Lenin: On the Slogan for a United States of Europe; in: LCW Vol. 21, pp.341-342 (Emphasize in the original)
 V. I. Lenin: On the Slogan for a United States of Europe; in: LCW Vol. 21, p.340
 V. I. Lenin: Imperialism and the Split in Socialism; in: LCW Vol. 23, p.110 (Emphasize in the original)
 Leon Trotsky: Is the Slogan “The United States of Europe” a Timely One? (1923), in: Leon Trotsky: The First Five Years of the Communist International, Vol. 2, New Park Publications, London 1974, p. 344
 Leon Trotsky: Disarmament and the United States of Europe (1929), in: Trotsky Writings 1929, p. 357
 Imperialist War and the Proletarian World Revolution. Manifesto of the Fourth International adopted by the Emergency Conference of the Fourth International, May 19-26, 1940; in: Documents of the Fourth International. The Formative Years (1933-40), New York 1973, pp. 324-325