The history of this is long and sorted but before I get to that it should be noted that it was many years before the country decided to amend the constitution again it goes without saying that this idea of amending the constitution seems to gain force the more time goes by.
The issue that brings this up is of course for many years slavery was still legal in this country after the passing the Constitution, It probably goes without saying that a large and very costly Civil War was fought, IMHO, of succession from the union but slavery was most certainly a catalyst issue for both side. That said this amendment was initially proposed while the Civil War was still going on in December of 1863.
The original proposal read this way:
All persons are equal before the law, so that no person can hold another as a slave; and the Congress shall have power to make all laws necessary and proper to carry this declaration into effect everywhere in the United States
It should be noted that there was a fight about this. Mostly this was due to the fact that southern states were not going to be present for this debate because the War was going on. This meant that the votes went 38-6 in the Senate and 93-65 in the house which means the House lacked the necessary two-thirds to make it a reality in February of 1864. It should be noted that only four Democrats voted for it and the Republicans basically sponsored and pushed the Amendment. This became more apparent when Abraham Lincoln during the presidential race of 1864 pushed to have the passage of the 13th amendment as part of the party platform. In January of 1865 the Bill although modified to include the idea the slavery could be used as punishment for a crime. Passed but that does not end the story.
The state ratification process took much longer and it went well on into 1865 after the war was over, It officially became part of the Constitution in December of 1865. It is interesting that there are a couple of states that took a long time to ratify this at this point and in part it was probably because those states considered the fight over so what was the point. Delaware waited till 1901, Kentucky 1976 and Mississippi 1995. Pretty much a perfunctory mover for everyone after December 1865.
If I were to give an opinion to me this idea of a lack of slavery should be evident in the plain words of the constitution but what constituted a citizen and a human being was in question for a long time in this country. In the end this amendment became necessary to basically say the institution of slavery is not accept in the United States – period. The only thing the that is the exception is the idea that slavery could be used as punishment for a crime and that was probably to protect the idea that inmates on prisons are sued as a work force and often for free.
As a Libertarian, I can applaud this amendment for making it clear that slavery is unacceptable, it is just a shame that it took some much conflict and death to bring about something that should have been self-evident about all human beings. The thing was though that the fight was not over. With the return of southern legislatures to both the house and senate other rights had to be upheld and this required two more amendments to the constitution.