A fundamental dilemma of foreign policy that we have (it can be argued) inherited from the Puritans is the idea that we should be a "city upon a hill," an example to the rest of the world.
What this often seems to mean to us is that we need to reform the rest of the world. We feel the need to try to make them in our image, to ensure that they will become democratic like we are. The dilemma comes from A) the fact that there are so many countries that need reforming and B) the fact that it is very hard to make countries become like us.
We do not have the money or other resources needed to reform every country in the world and we have seen time and again (Iraq, Afghanistan, Vietnam) that we cannot cause other countries to become like us very easily. Yet we still want to try because of our feeling that we are the city upon the hill. This is a fundamental dilemma that we have inherited from the Puritans.