We could say that they were:
1) the end of slavery, layperson of this fact, we can see how many freed slaves went west to seek a future and improve their quality of life
2) The gold rush, this fact marked a great point in the history of the country, since it was an engine to populate the west of the country, either with people of American nationality and people from other nations
3) The railroad, this means of transportation helped in a positive way the movement of people and goods throughout the country, making contact between peoples more fluid.