In an essay entitled "Two Americas" Walter Karp puts his finger on what seems to be the most important divide in the United States, the divide between being a "republic" or being a "nation." As he puts it there: "...there are two distinct Americas, two separate objects of the patriot's devotion, two distinct foundations of two contrary codes of political virtue. One is the American nation, the other is the American republic. At every important juncture of our political life these two Americas conflict with each other." [p. 13, in Buried Alive, a book of essays by Walter Karp published after his death.]
As Karp points out, when the New York Times published the Pentagon Papers and were faced with the government's attempt to censor that publication, the law firm that had represented the Times for several decades refused to defend the Times because to do so in wartime would be unpatriotic. But this is only consistent with one version of patriotism, viz., "the corporate entity known as the nation."  "To defend the infringement of liberty, to refuse to uphold the Constitution in a crisis, to support alien methods of despotism - surely that in a republic is shameful, disgraceful, and unpatriotic. The nation pulls one way, the republic another. They are today deadly rivals for the love and loyalty of the American people." 
Further, as Karp points out, "Americans are not fellow nationals, we are fellow citizens." And "America is more a creed than a country, and the creed is republicanism. The ties of a common nationality do not bind Americans together and never did."  This devotion to nationalism or as Karp calls it, "nationism," is much newer than our republicanism, traceable to the late 1800s when the United States undertook what Henry Cabot Lodge called the "large policy," that is, an interventionist, imperialistic foreign policy which led to the acquisition of the Philippines, Hawaii, and repeated occupations of Cuba. "The cult of the nation" requires such a foreign policy because it is only in the international arena that the abstract thing, the nation, comes to life. We were as a nation, well into our second century, before the cult of the nation required a cult of the flag and a cult of militarism. There was no pledge of allegiance uttered in any classroom before 1892 and the elaborate rituals involving our flag were only created by the War Department and the American Legion in 1923. As Karp wrote: "The object of the flag code was to transform the country's banner into a semi-holy talisman ans so give the abstraction called a nation a semblance of life."  And, of course, the nation requires what is called "a strong sense of international duty," which means ultimately, a sense that we as a nation must not only be prepared to make war but be willing, even eager, to make war. After all, war is the most visible action a nation can take. And war would underline that sacrifice, even the sacrifice of one's life, is or should be among the most important political virtues. As JFK said, "Ask not what your country can do for you but ask what you can do for your country." Note well: JFK could not have logically said this about a republic. In these ways, "A 'new religion' of nationism eclipse[d] and even supplant[ed] the old republican patriotism." 
There is also another way of looking at or labeling this great divide, viz., by distinguishing between a "union" and a "nation." To indicate briefly the differences here, in a union, such as a marital union, the parts do not or should not lose their integrity, whereas in a nation, the parts should lose their integrity and be subsumed by the whole. In a nation, the whole subsumes of consumes the parts, whereas in a union, it is important to preserve the parts as they are what make a union a union. A union is impossible without diversity, diverse parts, whereas a nation is impossible with diversity. Where unions strive for diversity, nations strive for homogeneity.