War brings Americans together, as Stanley Hauerwas and Martin Marty have pointed out. Our great wars – Independence, the World Wars, and so on – have been an opportunity for Americans to join together and create a common bond. (I think you could argue that the Civil War doesn't function quite as well here, since it was so terribly divisive and violent. Lincoln argued for holding the Union together, but it still showed a deep split in the American psyche, where the enemy was not outside, but within.)
But another important factor in American unity is a common enemy. In the 1700s and 1800s that enemy was Catholicism, which was European, hierarchical, and other. John Adams, for example, commented that Catholicism was "Hindu and cabalistic" (Marty, Pilgrims in their Own Land, 141). That enemy has also been Asians, as in California exclusion laws in the 1800s (there was "an irrepressible conflict" between America and Chinese culture, Marty, 262), Communists, Jews, Hispanics, and Muslims. The other as enemy has lurked as a strange, unifying factor in American history.
One of the lurking questions for studying religion in America is this question of unity. How does a nation come together, despite being, from our very founding, so disparate in beliefs? It is a question that scholars raise more often than answer. Perhaps there are no easy answers. We can point to Protestant revivalism, civil religion (the belief in America itself), war, belief in human rights, and a common enemy as important elements of American unity. But these can seem weak before a nation that is also parochial, tribal, and divided; Hauerwas' concern that our politics remain local doesn't really answer the larger question of national unity.
I would suspect that war and a common enemy have always defined nations and peoples in deep ways. What is different is a nation as disparate as the United States that has maintained a kind of unity despite its lurking and continuous divisions.