After the assassination of U.S. President Abraham Lincoln on April 14, 1865, Vice President Andrew Johnson became President. At first, he appeared to be a Radical however he clashed with them, involving himself in a bitter struggle with Radical Republicans.
Some of the main reasons which caused the political conflict between the radical republicans and President Johnson were the Civil Rights Act, which was the first United States federal law to define citizenship and affirm that all citizens are equally protected by the law, and the four Reconstruction Acts, through which it was required to the Southern States to be readmitted to the Union.
Since Republican Radicals had full control of Congress they were able to override 15 of the 21 bills passed by Congress which Johnson had vetoed, including the Civil Rights Act of 1866 and four Reconstruction Acts, described above.