1 The reconstructed state governments were required to denounce secession and ratify the Thirteenth Amendment (the abolishment of slavery). Many southern states refused to ratify the amendment and enforced black codes.
#2 The states had to submit to military law
In March 1867, Congress passed the Reconstruction Act of 1867 over Johnson’s veto, which invalidated state governments formed under presidential Reconstruction and imposed martial law on the ex-Confederate states. Only Tennessee, which had ratified the Fourteenth Amendment, escaped invalidation and military subjugation. The other ten states were reorganized into five military districts run by Union generals.
#3 The states had to ratify the 14th Amendment to be accepted. (The amendment requires states to provide equal protection under the law to all persons (not only to citizens) within their jurisdictions)
Southern states were required to ratify the Fourteenth Amendment in order to be eligible for readmission into the Union. In June 1868, seven ex-Confederate states voted to ratify the amendment, and the amendment finally passed.
# 4 The states were required to ratify the 15th Amendment to be re-admitted.
The Fifteenth Amendment, proposed in 1869 and passed in 1870, guaranteed the right to vote to any citizen regardless of race, color, or previous condition of servitude. The amendment aimed to promote black suffrage in the South, and to guarantee it in the North and West. (Much of the North had not yet extended suffrage to blacks, even though the South had been required to do so by Congress.) The last Southern states awaiting readmission—Texas, Mississippi, and Virginia—were required to ratify the new amendment as a precondition for readmission.