Throughout Africa and the Middle East we see countries with straight borders that used to be colonies of Western powers. These powers left and left behind democratic governments. These governments are plagued by the fact that the countries they rule over are not nations.
A country is a political entity - a set of borders enforced by an army. A nation is a psycho-social entity. It is a group that people identify with, normally composed based on ethnic, linguistic, or religious similarity. Nationhood marks an in-group. Americans have an anti-nationalist, universalist ideology, so their brains reject this reality to the detriment of the places their armies occupy.
In most post-colonial countries, there are many different nations within an arbitrary border. Democracy does not work because the citizens don't see themselves as the same people. When a person gets power in the government, of course he turns into a "kleptocrat" instead of working for the common good because he doesn't value the "common good". He values the good of his people.
These post-colonial countries are also prone to civil war. If one ethno-religious group gets the majority over another, the losing group doesn't see the government as legitimate. So there is war and genocide. The post-colonial idealists in the West have done more damage to the world than the colonialists ever did through the proxy of civil war.
Democracy in the Congo led to decades of Civil War and now we even see Bantus eating Pygmies[1] - rape, disfigurement, and even cannibalism have become weapons in a brutal civil war. Likewise, if American troops leave Afghanistan, we will see a recap of the 90's civil war between the northern and southern ethnic groups.
There is no "Afghan" people - there are Pashtun and Tajik and Uzbek[2]. And they were on the tail-end of a civil war before the USA invaded, and now the USA wants them to vote together. What insanity. In an ominous sign, no Pashtun will join the "national" Afghan army. Putting people within the same borders and putting a single army in control over them doesn't magically make them care about each other.
This is leaving behind the absurdity of trying to impose Western norms on an African continent lacking thousands of years of western history with literature, law, and democracy. You don't get to Tom Paine without Cato - and there was a 1700 year process connecting the two. Western thinking about the rest of the world is full of unicorns and pixie dust.
[1] http://www.theguardian.com/world/2003/jan/09/congo.jamesasti...
[2] http://origins.osu.edu/sites/default/files/2-12-map586.jpg