Even if a communist can colloquially describe themselves as being on the left, there’s a distinction between communism and “the left.” This is implied right in the title of Lenin’s Left-Wing Communism: an Infantile Disorder. Whereas the left, a big tent term for a myriad of incompatible ideologies, aims merely to act as an opposition towards the present order for the sake of it, communists have a coherent vision for how to defeat the system: by advancing history’s development to the next stage. The left, because of its lack of commitment to that central Marxist goal, naturally takes on an opportunistic role. Because when you want only to build a movement as an end in itself, rather than use this movement as a means for defeating the system, you become nothing more than an actor who benefits from discontent without helping solve the problems behind that discontent.
There is one point i would correct you on and that’s the notion that the reason why bourgeois revolutions were seen by the early Marxists as progressive is because they led to “political emancipation”. From my understanding that is not what Marx meant by progress. The bourgeois revolutions were progressive in the sense that they emancipated the productive forces of society. Capitalism allowed for higher development of productive forces than feudalism did. Socialism is progressive because, as Deng said, it enables “faster and greater development of those forces than under the capitalist system”.
But Capital was already free, hence why the colonies started in the first place. This was Capital overthrowing the national barrier. The British bourgeoisie and extant aristocracy did not have an interest in letting the settlers conquer the whole continent, fearing they would overshadow the economy of the Isles.
I think it’s better to look at the American War as an intra-bourgeois conflict between the high bourgeoisie in England and Scotland and the middle classes who fled to the Americas to escape being accumulated and proletarianized.
Britain was entering the terminal phases of monopoly and financialization, hence the expanding need for new territories and pushing production to America, Germany, and later Russia. These economies soon became a threat to Britain’s Imperialism hence the high taxes and territorial limits on the Americans and a century later the world wars to slow down Germany and Russia.
What the American settlers ask for again is a new frontier to open by removing the Monopoly order ruling America, but not necessarily through the people taking control of Capital (Socialism) but a reform of the property order.
For describing what was happening at the time, it was the progress of capitalism maturing, based on the material conditions and over time we increased what humanity was capable of. Was the slavery necessary? No, and the civil war allowed the industrialists to move south and helped capital expand more. Was the genocide necessary? Probably not, but that alternate timeline would have had white and indigenous people working the goldmines and striking together against indigenous capitalists. We are not living at that time or timeline and can recognize that we need to make amends to our black and Indian brothers and sisters.
I think what the mixup is is that you are viewing things in terms of conflict as opposed to the further development of the productive forces of society. Slavery and Feudalism were overthrown because they could no longer satisfy the needs of society, and so the developmental forces had to evolve.