Final answer:
The crucial provision of the Military Reconstruction Act was enabling black men's suffrage, leading to African American political participation and the transformation of the South's political structure. This was cemented by the Fifteenth Amendment, which protected voting rights regardless of race.
Step-by-step explanation:
The most important provision in the Military Reconstruction Act for African American political equality in the South was the requirement that the former Confederate states approve new constitutions which allowed black men to vote. This provision was enforced through the establishment of military districts and martial law, ensuring the protection of freed people and overseeing elections. The registration of voters and the subsequent rewriting of state constitutions under military supervision led to significant political changes including the election of black politicians and the ratification of the Fifteenth Amendment, which guaranteed that the right to vote could not be denied on the basis of race or previous condition of servitude.
The Military Reconstruction Act, along with subsequent legislation pushed by Radical Republicans, brought about the first moment of mass democratic participation for African Americans and a transformation in the political landscape of the South. However, once military rule ended, the Reconstruction period saw a regression in political equality due to increasing white resistance, leading to the failure of Reconstruction's primary objective of creating an interracial democracy with equal rights for all.