I would have to go with Completion of the transcontinental railroad in 1869
The 3/5 compromise. Every one slave was worth 3/5 of a white man. So, two slaves would be equal to 1 1/5 white men.
The holocaust was a time period when a guy named Adolf Hitler decided to kill a lot of jews in Germany by sending them to concentration camps where they lived whatever time they had left very inhumane
I would say a diamond border