http://www.pbs.org/wnet/jimcrow/stories_events_enforce.html
The Enforcement Act, 1870-1871
Between 1870 and 1871 Congress passed the Enforcement Acts -- criminal codes that protected blacks' right to vote, hold office, serve on juries, and receive equal protection of laws. If the states failed to act, the laws allowed the federal government to intervene. The target of the acts was the Ku Klux Klan, whose members were murdering many blacks and some whites because they voted, held office, or were involved with schools. 

Many states were afraid to take strong action against the Klan either because the political leaders sympathized with the Klan, were members, or because they were too weak to act. A number of Republican governors were afraid of sending black militia troops to fight the Klan for fear of triggering a race war. But once Congress passed the Enforcement Acts, the situation shifted. One of the Acts, the Ku Klux Klan Act of 1871, made private criminal acts federal crimes; consequently, President Grant decreed that "insurgents were in rebellion against the authority of the United States." He sent federal troops to restore law and order to many areas where violence was raging at its worst. 

In nine counties of South Carolina, martial law was declared and Klansmen were tried before predominantly black juries. Much of the credit for prosecuting the Klan belonged to Amos Ackerman, Grant's Attorney General, who did his best to make the country aware of the extent of Klan violence. As a result of his efforts, a few hundred Klansmen were tried and sent to jail. Thousands of others fled or were let off with fines or warnings. By 1872, the Klan as an organization was broken. By the time the terror ended, thousands of blacks and hundreds of whites had been massacred or driven from their homes and communities. For a moment, it seemed that peace and Republican rule was restored. Yet within a few years, the terror was reborn and Reconstruction officially ended. 

-- Richard Wormser