Marx struggled quite a bit with this. The basic assumption of communism is that there's a single, identical, collective interest of the global proletariat by virtue of their relation to capital (as propertyless individuals). The big question for communists then is why isn't the proletariat realising this collective international interest and why aren't they organising as such to pursue their interests as workers. In other words, why aren't they just thinking Rationally Enough to realise it? One problem for Marx was that the English and the Irish proletariat didn't unite or work together, mainly because the English proletariat sided with the empire and wasn't too keen to accept Irish nationalism and saw the Irish as competitors, and the Irish proletariat was on the other hand not too keen about being part of the British Empire. He outright says he expected them to work together, which then didn't happen. Look how he describes in a letter (first pic) to Engels that Irish needs to emancipate itself from the working class, so that the working class focuses on its struggle as the working class, but that he "cannot tell the working class" themselves. Basically, he, Marx, as the great intellectual, knows better than the working class themselves what's good for them, and his job now is to manipulate things in a way that is good for the working class themselves, even if they themselves are not aware of it. Basically he wants to be the catalyst for Hegel's Cunning of Reason. There's constantly all these factors that supposedly prevent the working class from thinking Rationally and doing what's Right (according to Marx). The classic "billionaires tell the English to hate the Irish to divide them" trope was actually thought of by Marx all the way in 1870. The history of communism is just a long, very long list of copes to try to explain why the proletariat is not thinking rationally and isn't coming to the "correct" conclusion of communist revolution. From Stalinist purges and de-kulakisation, to the Maoist cultural revolution, to Adorno and Horkheimer's Culture Industry, to Gramsci's "cultural hegemony"... it's all this. Over and over and over. They simply do not want to accept that there could be genuinely rational and legitimate reasons to not do communist revolutions and build "international solidarity".