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People have learned by bitter experience that the "European fraternal union of peoples" cannot be achieved by mere phrases and pious wishes, but only by profound revolutions and bloody struggles; they have learned that the question is not that of a fraternal union of all European peoples under a single republican flag, but of an alliance of the revolutionary peoples against the counter-revolutionary peoples, an alliance which comes into being not on paper, but only on the battlefield.

Karl Marx, Friedrich Engels (1975). “Karl Marx, Frederick Engels: collected works”, Intl Pub
People have learned by bitter experience that the European fraternal union of peoples cannot be achieved by mere phrases and pious wishes, but only by profound revolutions and bloody struggles; they have learned that