The greatest issue facing Andrew Johnson immediately after being sworn in as President after the death of Abraham Lincoln was, of course, the reconstruction of the war-torn southern states - how to rebuild their infrastructure, what to do with those who had fought against the north and how to deal with the newly-freed black population.
For the first several months of his administration, Johnson had the issue to himself, as congress was in recess until that December. The President took this time to issue a sweeping reconstruction agenda based on both his southern sympathies (which included a personal racism which resulted in continuing to deny blacks many rights) and his constitutional belief in preserving the union.
Johnson pardoned many in the south who formally pledged allegiance to the union and renounced the confederacy, and appointed provisional governments to help get the states running again.
When congress convened (controlled mostly by northern Republicans), they immediately began to clash with the President. Congress supported granting more rights to blacks, and refused to support Johnson's appointments to southern state governments. Johnson even vetoed a revolutionary civil rights bill in 1866 to granted unprecedented rights of citizenship to blacks, though congress (for the first time in history) overrode the veto.
Johnson's Downfall
After Republicans won two-thirds of the congressional elections in 1866, they had even greater power over the President. They proceeded to oversee reconstruction in their own way, imposing martial law into much of the south (including Johnson's home state of Tennessee).
Little by little, under congressional control and being led mostly by northern republican "carpetbaggers" or southern "scalawags," the southern states began to have constitutional conventions and be readmitted to the union as states. This was in direct opposition to Johnson's belief that the constitution itself did not allow for secession, and thus the southern states had never technically left the union in the first place.
In 1867, attempting to further curtail the President's power, congress issued to acts - The Command of the Army Act, which forced Johnson to issue orders only through Ulysses S. Grant, the General of the Army (who could not be removed without Senatorial approval) and the Tenure of Office Act, which refused to allow the President to fire certain appointed officials without Congressional approval.
Backed into a corner in facing a congress that was too powerful for him to overcome, Johnson rebelled against their new laws, responding by dismissing Secretary of War Edwin Stanton in August of that same year while congress was in recess. After they had come back in session, Johnson tried to explain his reasoning to them, but they refused to allow the dismissal and Secretary Stanton was reinstated.
A year later, Johnson tried once again to fire Stanton, but this time, congress took the matter even more seriously. The House of Representatives quickly, and without any hearings or testimonies, voted overwhelmingly to impeach the President, both for ignoring the Tenure in Office Act, as well as for bringing what they saw as disgrace on the office of the Presidency.
Though the measure failed in the Senate by only a single vote, Johnson became the first President (and only one of two to date) to have been impeached by Congress.
Johnson's Legacy
In the end, Johnson's partisanship and refusal to work with a congress that opposed him (as well as similar stubbornness on the part of congress) became the primary reasons for his political downfall.
Johnson tried to receive the democratic nomination for President in 1868, but his party, for obvious reasons, were unwilling to nominate an impeached, failed President. They ran Horatio Seymore, instead, who lost handily to the war hero Ulysses S. Grant.
After returning to Tennessee, Johnson continued to remain politically relevant, running failed elections for both the Senate and the House of Representatives before finally being "vindicated" by being elected to the Senate in 1875 - though he died only months into his term (he remains the only former President to return to the Senate).
Andrew Johnson is generally regarded as one of the least successful Presidents in American History - a man who was a decent politician outside of the highest office, though who was not at all qualified to face the intense difficulties facing the nation after the Civil War.
Especially problematic for Johnson was his attempt to follow Lincoln, who is generally regarded as being one of the great leaders the nation has ever seen. Was Lincoln to survive, reconstruction surely would have gone differently, though historians are divided as to how exactly this might have played out.
References:
"Andrew Johnson." American Presidents: An Online Reference Resource.
Articles of Impeachment for Andrew Johnson.
"Biography of Andrew Johnson." U.S. Senate.
