In a letter to the editor of The Ventura County Star for February 15, 2017, Ray Sobrino Jr. of Newbury Park, CA, ended with the admonishment to "remember we are all Americans in the end." I won't argue with the letter or even the sentiment of this clause, but I will reword it to, "we are all Americans if you pull back and take a long-distance view of ourselves" — because close up we see important differences.
One set of differences is political, and nowadays that's pretty obvious with continuing support for Donald Trump from most of the 46.1% of voters who votedfor him and continuing opposition from many of the 48.2% who voted for Hillary Clinton — and at least some of the nearly 6% of the electorate who voted for someone else and the large number of people who just didn't vote.
More important, there are deep and long-stand differences you can analyze in many ways, but my current favorite is Colin Woodard's American Nations: A History of the Eleven Rival Regional Cultures of North America (2014) and the idea that there is no single American nation but either none or up to eleven, depending on how you define "nation."
Most of us on the territory of the United States are citizens of the American Republic, and we mostly agree on loyalty to the Republic. What holds us together is our agreeing to argue over what the Republic should be, who should be members, and what sort of society we should try to build.
"We are all Americans" — and then we fight over what that should mean.