This set of files shows the configuration of United States and Territories every 10 years from 1790 to 1920. You can watch the United States start out with the 13 original States (actually I think it was 14 by 1790) and expand westward to the Pacific. I believe the only thing to change after 1920 was Alaska and Hawaii being changed from Territories to States, which is why I stopped at 1920.
The original shapefile data was obtained from the National Historical Geographic Information System. I simplified the polygons to allow them to display in Google Earth.
Each decade is an approximately 1 to 1.5 megabyte file that will take a minute or so to download and be processed by Google Earth, depending on the speed of your Internet connection and computer.