The first school in the US was Boston Latin School. It was a one room school house. This school started in 1635. This event is important because it started the education in continental America. It started when France owned most of North America.
World War One (1914-1918) revolutionized military equipment for the nearly every country in the world.
World War Two (1939-1945) helped shape religion, economics, etc. Even though the US joined in 1941, the things that happened in 1939 and 1940 is still important.
Even though this site was Wikipedia, I still checked the information (all of it). Football started in 1869. Flags started trickling in after 1870. This event is important because football is arguably the most popular American sport.
The Great Depression (1929-1939) is important because it (in the end) helped American businesses.