Ask me that question in 1789 and I'd have said these silly 13 colonies were too different, too hostile, with too many competing interests to form a nation, and that the people trying to make it happen were amateurs and incompetents. Which most of them were, after all...
All I can say is that sometimes democracy looks like this. The whole Iraq thing could fall apart tomorrow...but it hasn't yet.
in all fairness, iraq is being held together by external political design and OIL. As an ethnic entity, Iraq would make 3 separate countries and if allowed and the oil revenue control question were not extant I bet majorites of all 3 ethnic groups would elect to separate. Honestly I see no compelling internal reason to force them to be a single country.
History will judge how creating a shiite-majority government in iraq to replace hussein works as a trade-off, particularly given the iranian nuclear program being close to fruition (months, years? who knows).
Once you get down to the hypothetical question of who will control and/or administer the oil fields, funds, etc., in a breakup then there is an entirely different element to this picture.