To put it very simply, Marx's internationalism supported movements against colonialism and imperialism. He believed the proletariat had more in common with the proletariat of other countries, and called upon them all to work together. This, however, as understood by Marx meant supporting the movement of Ireland as a whole against the British Empire, of Poland against the Russian Empire. He correctly understood that without combatting colonialism, the proletariat of oppressor and oppressed countries could not be free. The quest for national sovereignty is principally an anti-colonial movement.

The abolition of borders is not the same as the abolition of nations. Marx never stated that the nation would wither, but that states would. He did not believe nationality to be a "bourgeois construct." The state, borders, all of this withers as society gradually collectivizes across the entire globe, in international revolution, according to Marx and Engels alone. At this point, no developed theories of nations or imperialism or self-determination exists yet as core to Marxism-Leninism, but even within Marx himself there is a strong understanding that colonialism is a barrier to proletarian revolution, not merely capitalism at a state level.

"Workers of all countries, unite!" The international struggle is one that combats the bourgeoisie of all states, of all nations. However, it principally fights the colonizing bourgeoisie as the primary antagonist when this is a barrier to revolution, as it was in Poland and Ireland. This is the national question before Lenin and Stalin.

Any condescension you see on my part is a response to the repeated, incorrect claims that I am revising Marx. Reading Marx's work on Ireland and Poland make it clear that Marx understood colonialism to be a core obstacle, and thus simply saying "fight the bourgeoisie" without any nuance to it would be reductive and vulgarize Marx. This is why Marx supported the Statesian north against the south, despite both sides being bourgeois.