President Lincoln.
Mr. Lincoln had a great love for the United States Constitution. i.e. The wisdom in the words, and the power of an idea on how people should govern themselves and that is, freely. And also how humanity should respect itself and treat others. It's what separates us from the cave people from millions of years ago of the select few making all the decisions for the rest of us.
If President Lincoln just let the South go and did not hold those ideas together for the next generation, then they would have fallen right back into the dark ages. Why not just send a letter to the Queen of England, apologize for the trouble they caused, and asked to be taken back?
The United States Constitution was still so very new in the 1860's - The idea of one nation treating all it's citizens the same, and that we do not divide ourselves into classes. Europeans measured their histories in centuries. The system of the Chinese has been around for thousands of years. It was no shock that the Union, the precious child of a country, was having so many problems in the 1860's. Again, there was nothing to guide them, and no example for them to follow. There was no one to turn to other then each other. The new idea of the United States of America and the U.S. Constitution had not even around for one hundred years during the American Civil War.
If the rebellion was successful, I believe history would have said about the U.S. Constitution is that Americans at the time were arrogant and had not earned any respect, which is why the American Union did not last more then one hundred years, and no one would have ever taken the idea the U.S. Constitution seriously. It would have been presented as a joke in the history books in my view.
BUT, since President Lincoln did end the rebellion and brought the country back together again and proved the system can work, we became a threat to the tyrants of the world because we inspired most of the rest of the world to live by the same standards. Nothing stopped others from allowing their countries to give the people the freedom to think for themselves. People realized that they did not have to suffer under someone else's domination. They used the U.S.A. as inspiration and our system spread all over the world and has become a better place as a result.
When the Union was born in 1776 and then fully preserved, in 1865 it was every Kings, Monarchs, and Despots worst nightmare. They started looking out their windows for the next John Adams or Ben Franklin or George Washington to rise up out of the muddy fields of the oppression of some small village and sweep them away.
If the Union was not saved, and was not able to show the world the system does work, and that we can build a nation and manage our affairs from the power of an idea that written on a piece of paper...then that idea would have died. If the Union was not saved, something of great value would have been lost with it.
History would have recorded that the idea did not work, that our piece of paper did not carry the power of a monarchy, the Constitution was not efficient as the power of an elite ruling class, and that it is acceptable for one class of human being to posses and dominate another. There is a significance to this that goes far beyond our borders, and far beyond our time.
President Lincoln understood the absolute necessity to make an abstract ideal created by our Founding Fathers real. No mean little feat by any means. The words of the the Constitution and the Declaration of Independence are dead as door nails on the page unless living human beings act on them. The nation HAD to ACT to make those words manifest. That was the fight. And it was one hellova fight.