Oh, and to add my own comment here...
Slavery. And I don't just mean the decision to permit slavery to continue existing within the US constitution. I mean the moment the first abducted African was brought to our shores. The practice of slavery, especially racially based slavery, is unabashedly evil. And it was the single most dividing issue in our history. Without slavery, there would not have been a civil war, nor the continuing animosity between north and south that continues to this day. The incredible racism that the US has against blacks would never have existed, and we would be a more moral people for it. There would be no KKK, no lynchings, no Jim Crow. There would be no war on drugs, since a great deal of the drug enforcement policies are aimed specifically at investigating and targeting black drug users, despite drug use being relatively equal between middle-class, suburban whites, and poorer blacks.
A lot of our conservative and liberal conflict would not be here without slavery. The basic divide between liberal, city-dwelling Northerners and conservative, country-dwelling Southerners would never have happened. The notion that we should be split into two polarized groups would not have occurred. We would not have such strong political parties, if any at all, without slavery to divide us.
We would not have this partisan divide, we would not have had one of the bloodiest wars in history, we would not have a history filled with racism and violence against blacks. We would never have had the conflict that caused such unilateral power to be concentrated with the federal government.
In every way, we would be a more moral and freer people. All we had to do was not abduct people (or purchase people abducted by their neighbors), ship them across an ocean, and literally work them to death, and then do it again to their children.