Reductionism is not about reducing the importance of a thing. It’s about reducing everything else so only that thing matters. This is why identitarians often accuse socialists like Adolph Reed of being class reductionists. I prioritize class just as Du Bois and King did, yet people who erase the socialism of Du Bois and King say I only care about class—and miss the fact that the working class is more racially diverse than the owning class, so even if class reductionists existed, they would do more to help black people than race reductionists like DiAngelo and Kendi do.
As for the Civil War, as the war proceeded, more and more of the people fighting shared the goals of the abolitionists. If that had not been true, slavery would not have been legally abolished afterward.