Look, I'm always down for learning more about history, but who's "Nigeria"? To who was "Nigeria" selling slaves to? Modern states are never representative of specific / homogeneous cultures, let alone individual
peoples, let alone societies from before the state was even formed. After skimming a few wiki articles, it's clear that the region has had its own fair share of struggles against authorities, slavery, and racism, even before European colonisation, some of which continue currently.
Still, none of this reached the scale of european colonisation / "the scramble for Africa", and the continued political and economic influence and control that 'the West' continues to hold and wield (neocolonialism / recolonisation). I know nowhere near enough about critical theory, but I'm sure these processes can be understood as a form of deterritorialisation and reterritorialisation (I wasn't able to find a freely available copy, but this article seems like it could be a relevant, interesting read: Deterritorialization and Reterritorialization of the Orisha Religion in Africa and the New World (Nigeria, Cuba and the United States).)
Regarding Syria, "my approach" would be simply to support more movements / projects like Rojava (which is clearly not something that 'western' political powers are interested in doing). As an anarchist I don't think liberation from state authority can be achieved through state authority.