Grant generally sided with the Radical Republicans after the Civil War. He had a personal spat with Andrew Johnson due to his mishandling of Edwin Stanton during the controversy over the Tenure of Office Act, as Grant would not willingly replace Stanton when he was fired by Johnson.
Grant viewed Reconstruction as a civil rights issue. Grant used federal authority to break the power of the Ku Klux Klan in the South, and his law enforcement officials won convictions against over five hundred members of the Klan, thus breaking their power in the South. Even though this would prove to be relatively short-lived, this was important, as the Klan was a growing vigilante and terrorist organization which controlled government and law enforcement in many Southern towns.
Grant was also instrumental in getting the Fifteenth Amendment passed in 1870, granting African Americans the right to vote. He, along with the other Radical Republicans, viewed citizenship as worth less if it did not come with suffrage. Also, some Radical Republicans viewed the newly-enfranchised freedmen as a power base for their party which would counteract the Democrats.