After the Civil War radical Republicans believed that the Southern states were "conquered provinces" that had completely left the Union and were therefore at the mercy of Congress for readmission.

True.

Explanation:

Not only democrats but there were also people within the republicans who believed that Lincoln's plan of reconstruction is too forgiving for those who were responsible for the cause of war. One of the republicans member named Thaddeus Stevens even said that Southern states were conquered provinces. Radical republicans believed that men who were never part of the Confederacy should be allowed to take part in the new government.