In my opinion, the unique American nationality developed a great deal during the 18th century. Of all things, the French and Indian War (the 1750's) probably had one the greater effects on the feeling of nationality. For the first time, the colonists felt a feeling of military success. This undoubtedly fueled a sense of unity. Moreover, Enlightenment thinkers like Benjamin Franklin contributed to the "new" American culture. His developments as an American were unique and stimulating. I think that regional differences in the colonies continued to grow and grow until the outbreak of the Civil War in 1860. The South was able to grow a different and wider variety of things, thus creating its own economic identity (slavery, etc). The North remained more religious-affiliated and began to develop things like more universities, which the south generally did not (not a single ivy-league college in the south). A very different ideology continued to create each colony own identity in the 18th century and continued for quite some time.
