Nationalism does not help against oppression because you cannot use nationalism against capitalists as capitalism is not a nationality but the root cause of oppression itself.
Nationalism is a tool for the owning class, that is not itself affected by the circumstances that enable the becoming of nationalists and is simultaneously shielded from the fallout of nationalism that inevitably ensues, to further division between us humans and to give people an incentive to go to war against their brothers and sisters for them in their stead, and to spill their blood over a made up symbol that has no actual meaning.
Always conform, self must be denied in favor of the manufactured self