Marx to Kugelmann, 1870:

The average English worker hates the Irish worker as a competitor who lowers wages and the standard of life. [...] He regards him practically in the same way the poor whites in the southern states of North America regard the black slaves. This antagonism between the proletarians in England is artificially nourished and kept alive by the bourgeoisie. It knows that this split is the true secret of maintaining its power. [...] Finally, England today is seeing a repetition of what happened on a gigantic scale in ancient Rome. A nation that enslaves another forges its own chains.