Engels said in 1874, discussing Polish independence: “A people which oppresses another cannot emancipate itself.”  This line, and other similar ones by Marx and Engels, are celebrated for their incipient proletarian internationalism. However, they are often subtly misinterpreted, often narrowly understood in a moral sense, e.g. “If you harm others, you are also harming your own soul.”
Consider the next line in that original text, A Polish Proclamation:
The power which it uses to suppress the other finally always turns against itself. 
The correct way to understand this line of thought is that there is a dialectical material relationship between repression in the colonies and domestic repression. The brutality of the enforcers and the flexibility afforded to the bourgeoisie by all the surplus value extracted shapes and deforms all of society around it, both at home and abroad. The relationship is all-encompassing: the truth is the whole.
This idea was notably laid out by Aimé Césaire in his Discourse on Colonialism,
What [the white man] cannot forgive Hitler for is not the crime in itself, the crime against man, it is not the humiliation of man as such, it is the crime against the white man, the humiliation of the white man, and the fact that he applied to Europe colonialist procedures which until then had been reserved exclusively for the Arabs of Algeria, the “coolies” of India, and the “niggers” of Africa. 
Césaire brilliantly makes his case not primarily by citing fellow left-wing thinkers, but by citing French imperialists at great length, in their own words. This is always very compelling! Consider Friedrich Nietzsche in 1881, explaining how colonization and white supremacy could defuse the threat of a national proletarian revolution:
Everyone ought to say to himself: “better to go abroad, to seek to become master in new and savage regions of the world and above all master over myself; to keep moving from place to place for just as long as any sign of slavery seems to threaten me; to shun neither adventure nor war and, if the worst should come to the worst, to be prepared for death: all this rather than further to endure this indecent servitude, rather than to go on becoming soured and malicious and conspiratorial!” This would be the right attitude of mind: the workers of Europe ought henceforth to declare themselves as a class a human impossibility and not, as usually happens, only a somewhat harsh and inappropriate social arrangement […] Perhaps we shall also bring in numerous Chinese: and they will bring with them the modes of life and thought suitable to industrious ants. 
The enduring relevance of this line of thought is obvious: the people in a region like what we today call “Canada” will never have a socialist revolution until that entity loses all of the colonial interests (e.g. mines around the world) that allow its bourgeoisie to use its loot to shape the domestic power pyramid with bribes, and to safely employ its violent miscreants in white supremacist policing.
Consider Walter Rodney’s analysis from 1971:
European workers have paid a great price for the few material benefits which accrued to them as crumbs from the colonial table. The class in power controls the dissemination of information. The capitalists misinformed and miseducated workers in the metropoles to the point where they became allies in colonial exploitation. In accepting to be led like sheep, European workers were perpetuating their own enslavement to the capitalists. They ceased to seek political power and contented themselves with bargaining for small wage increases, which were usually counter-balanced by increased costs of living. They ceased to be creative and allowed bourgeois cultural decadence to overtake them all. They failed to exercise any independent judgment on the great issues of war and peace, and therefore ended up by slaughtering not only colonial peoples but also themselves. 
Our task is to make as many people as possible understand that anti-imperialism isn’t just the right thing to do, it’s also the smart thing to do. It’s fantastic whenever people find it within themselves to self-sacrifice based primarily on solidarity and moral indignation, but everyone should also understand that such sentiments are not and will never be sufficient to coordinate and make reliable the organization of the sheer numbers of people needed to wage a revolution. We must not only read but also share whatever relevant theory we can get our hands on.
One small caveat: We should not then slip into becoming one of those Spock-like academic Marxists that, as Nia Frome has pointed out, end up sounding like Watchmen’s “Dr. Manhattan” in their aloof anti-moralism.   Moral indignation is as much a part of reality as culture and religion. Such fire always plays an important role.
Aimé Césaire, 1955. Discourse on Colonialism. ↩
Walter Rodney, 1971. How Europe Underdeveloped Africa. ↩
Dr. Manhattan, at the same time he becomes a superhero with actual supernatural powers, becomes disconnected from humanity’s problems and indifferent to them, for they are all rational and predictable: “I’m tired of this Earth, these people. I’m tired of being caught in the tangle of their lives.” This is not a good attitude for a communist! [web] ↩