http://www.marxists.org/archive/lenin/works/1915/aug/23.htm
"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."
[...]
"United States of Europe under capitalism is tantamount to an agreement on the partition of colonies."
[...]
"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."
Hat tip to the Entdinglichung ("De-Reification") Blog for pointing this out: http://entdinglichung.wordpress.com/2011/11/14/lenin-zum-heutigen-tage/