Complex sentence: After the war, the Republican senators from Massachusetts, Charles Sumner and Henry Wilson, would transform their pre-war radical antislavery rhetoric into vehement support for radical Reconstruction of the South--an agenda that called for civil rights for African Americans and harsh treatment of former Confederates.
Split into simpler sentences: After the war, the Republican senators from Massachusetts, Charles Sumner and Henry Wilson, would transform their pre-war antislavery views into vehement support for so-called "Radical Reconstruction" of the South. Their agenda called for civil rights for African Americans and harsh treatment of former Confederates.