Jumping off from Zathras' article regarding the dearth of civility in contemporary America, I'd like to add a few thoughts of my own regarding the direction America is moving in.
As I noted in our first entry here, we may be standing at the door of a sea change in world history. The 20th century was indisputably the American century, ushered in by Teddy Roosevelt and the Great White Fleet, punctuated by the United States' balance-tipping entry into the First World War and America's central role in the century's second war to end all wars, and cemented with the decline of the Soviet Union and the emergence of the sole superpower. But the world is a dynamic place. Where America stood at the dawn of the 21st century, while differing in scope, was not all that different from where Greece and Rome and Britain stood at earlier times in human history. And each of those nations, in their turn, saw their influence ebb or disappear completely. While that does not mean that it is certain that America will follow a similar course, such an end does not seem implausible, either.
Sole powers are rarely popular, and with good reason. No matter how benign a hyperpower may wish to be, it will fail and its failures will generate resentment. Europe dislikes the U.S., among other reasons, because it is a painful reminder that Europe's time of world dominance has passed. Much of the Islamic world despises the U.S. because its culture undermines the foundations of their belief system. The Arab world hates the U.S. for its support of Israel and its perceived role in keeping the Palestinians subjugated. South America dislikes the U.S.'s historical penchant for interfering in their affairs. In many of these cases the U.S. can defend itself. The U.S. can hardly be blamed if other nations like American culture, for example, and the U.S. did not cause Europe to lose its relative influence over the rest of the globe. And when the U.S. has interfered overseas, it claims to do so for the best of reasons: good intentions. Such intentions are cold consolation for people whose families have been uprooted or killed in wars financed by American dollars, however, and many good intentions have ended in situations akin to what we now see in Iraq, where 26 million Iraqis live lives flavored with omnipresent fears of where and when the next bomb will explode.
Nor would isolationism solve America's problems, although it might well do less harm than America's current course. When nations are not cursing the United States for its interference, they are complaining that the U.S. is failing by not getting involved in some project or other; witness the common complaint held by many that the U.S. is at once wrong to be in Iraq and not to be in Darfur. The fact the United States possesses such great power relative to the rest of the globe means that people will generally want to see that power put to use for their pet causes. Recall Madeleine Albright's famous complaint of Colin Powell: what good is a great military if you can't use it? America's possession of a good military has led it into no small number of cases where it has chosen to rely on military force because it could. As a wise man once observed, when you're good with a hammer, every problem tends to look like a nail to you. So it is with American power: because it exists, people are going to want to see it used to further their own ends.
Add to this the changes that have reshaped American society over the past century. America was founded on the ideal of a place where people could come and be and do as they pleased. If you didn't like where you were, you could light out for the territories and live as you chose. The frontier closed down in the late nineteenth century, and American society had to change to deal with that. There was some degree of consensus for the first half of the century, albeit a consensus maintained primarily by the exclusion of nonwhites and females from power in almost every field. As other Americans were finally permitted to pursue that original American dream and more and more people shifted from relatively independent farms to cities where specialization and cooperation were required to survive, the consensus view of what America was or should be began to dissolve, and nothing has yet emerged to replace that.
Americans today are increasingly Americans only as an accident of geography. Even on September 11, 2001, when most Americans pulled together, if only briefly, in the wake of those attacks against them, filmmaker Michael Moore was asking why the terrorists had struck New York when New York was opposed to President Bush. In his logic, the ties that bound New Yorkers to the rest of the country were less important than their differences, and the terrorists should have realized that. Moore exists towards the extreme end of the spectrum, but his thinking is not irrational. Politically, the United States in many ways has split into two disparate nations (a tremendous oversimplification, of course, but bear with me) as we can see here. 
There remain points where we are all generally in agreement, but do they truly bind us together as Americans? In 2000 and 2004, leftists threatened to flee the country if the Republicans won, although few actually were willing to go that far. Right wingers seem less predisposed to that type of threat, but the rhetoric leading up to the 2006 elections was redolent with claims that voting Democratic was tantamount to handing the U.S. over to Osama bin Laden, a vicious claim that certainly implied that the Democrats were not fit to defend the country from its enemies, whether willfully or not. For a republic to survive in the long term, its citizens need to be confident that the system works; that whoever the electorate chooses to represent it, that person will be (on average) reasonably competent and will do a decent if not inspiring job. If enough people believe that is unlikely to happen, they will consider other methods to accomplish that goal. If enough Republicans and Democrats believe that the other party will produce leaders that will damage the country (and President Bush has done a great deal to justify that belief for Democrats), they will seek ways to ensure that those candidates cannot reach office. This is the justification for much of the electoral fraud seen around the world: the other guy would be a disaster, so it's ok to break the rules to make sure he doesn't get elected. It is a significant leap to decide that it is also ok to resort to violence to prevent such occurences, but that is merely a difference in scale once the decision it's ok to break the rules has been made.
America is not at that point yet. Despite President Bush's victory in 2004, the Democrats did not flee the country en masse. I am aware of no attempts to kill Nancy Pelosi or Harry Reid to derail the Democrats from pursuing their political agenda now that they are in the majority. Nor can I say with any certainty that this is necessarily going to change.
Despite that, I wonder just how long habit will out. Different parts of the country are growing less alike as time goes by. Having lived in many parts of America, I can attest that life is very different in different regions. American culture still exists, but it is a backdrop to the dominant culture in those areas, and it does not seem implausible that the people of America will grow further apart with time as those cultures continue to develop. As the federal government continues to grow in power, will there come a day when some subordinate elements of America decide that they no longer wish to have their lives controlled by distant populations with little chance for the locals to control their own destiny.
We have seen this before, of course. The American South broke away from the union in 1861 because they feared (probably rightly) that the federal government would force emancipation on them. That schism led to the greatest conflict yet fought in the western hemisphere, and the bloodiest conflict the United States has yet suffered. A future attempt to break away from the federal government might lead to the same thing, a far deadlier concept in an era when weapons far outstrip the destructive capabilities available from 1861-1865. Any such breakup, therefore, would need to be amicable if it were not to risk horrific devastation. Unfortunately, habit and fear of change would be likely to defeat any amicable moves towards separation; it would likely only occur if one side felt that it was the only option, and would then almost certainly lead to the federal government attempting to put it down violently once again.
Yet there is something perversely tempting about the idea of breaking the United States into a number of smaller states. Unlike 1861, there is no great moral issue that would be exacerbated by separation. Geography would ensure a great degree of cooperation among the new nations, which would probably retain some European Union-style agreements that would still allow people to travel without passports or undue paperwork. Smaller nations would allow people a greater degree of say in how their government ran. And an American military divided into four to six separate forces would no longer present such a tempting stick for leaders seeking to impose their preferences on faraway places, but would still be easily integrated if necessary for a truly defensive requirement.
I have no illusions about the likelihood of this occurring. American history has been one of greater centralized authority almost since the day the Constitution was adopted. I see little reason to expect that to change without a major upheaval. Which leaves unanswered the question of what may happen when that central authority does something people decide goes too far, an occurrence that is a matter of when, not if.