For starters the States never really had separate national identities among the original 13 so uniting is much simpler compared to the European nations who have

For starters the States never really had separate national identities among the original 13 so uniting is much simpler compared to the European nations who have a long history of different cultures and international competition. Then the rest of the States were people moving from the original 13 to new territories and creating states there (after massacring the native inhabitants of course) or the existing United States buying/conquering land.