During the Lewis and Clark expedition a whole new area was explored. They went where no one had any information just guesses and speculation. When they came back they brought back maps of discovered lands and waterways. People became more aware of the amount of land that was still to be settled and hugely thanks to the Lewis and Clark expedition, settlers were moving to the West.
The Western Migration was a vital event in American history and shaped the United States into what it is now. New cities and states were formed from what used to be unexplored areas. Interest in opportunity brought even more people to leave their homes and settle in the West. As a result the United States stretched to two borders.
The last event I thought was important in shaping the United States was the moon landing on July 20, 1969. Americans were pressured by the Soviet Union to be the first to land on the moon and discover what was earlier only dreamed of. When Neil Armstrong and Buzz Aldrin took a step on the moon, the United States influence increased even further. The Lewis and Clark expedition, Western Movement and Moon Landing were all vital towards expanding the United States and shaping it into what it is now.