Abraham Lincoln‘s assassination placed Andrew Johnson as the new US president. Johnson quickly started the process of Reconstruction while Congress was not in session.
Following Lincoln's plan, he pardoned most Southerners who took an oath of allegiance.
The southern states had to create new governments and ratify the 13th Amendment to the Constitution, which abolished slavery in the United States.
(The Emancipation Proclamation only freed slaves in the rebelling states, not the border states that remained loyal to the union.)
However, beginning with Mississippi, the new state legislatures passed harsh "Black Codes." The Black Codes were special laws that applied only to African-Americans. They aimed to provide cheaper labor and keep former slaves in a position very close to slavery. By the time Congress reconvened in December 1865, reconstruction had begun.
The Radical Republicans who controlled Congress wanted equal rights for the freed slaves. Johnson opposed this, but that did not stop Congress they passed the 14th Amendment which gave full citizenship to former slaves.
Congress also passed a series of Reconstruction Acts that placed stricter penalties on all the southern states, except Tennessee, which had already been readmitted to the union. It divided the south into five military districts under strict control. The southern states had to draft new state constitutions giving the right to vote to all adult males. Congress had to approve the new state constitutions.
Finally, the states had to ratify, or approve, the 14th amendment. Johnson angered the radical Republicans by using his veto power to try to block their harsh reconstruction plans.
On March 2, 1867, Congress overrode the president's veto and passed the Tenure of Office Act, which limited the president's ability to remove government officers.
In August, President Andrew Johnson tried to remove the Secretary of War, who opposed his reconstruction policies. For this, on February 24, 1868, the House of Representatives voted 126 to 47 to impeach Johnson.
Impeachment....
Chief Justice Salmon P. Chase presided over his trial in the Senate. He was saved by a single vote. During the remainder of his term as President, he granted amnesty to all the former rebels.