Security
When discussing the national security aspect of foreign policy, the first thing that comes to most peoples' minds today is the war in Iraq. That war is indeed the central issue of the day, however it cannot be looked at in isolation. The Iraq War is just one part of a larger issue, which is the global US military empire.
Since the Truman Doctrine was enacted six decades ago, our foreign policy has been predicated on the idea that America must spread democracy throughout the world, even if by force. This often gets us involved in attempting to resolve the disputes of others. The outcome of such policies is that we have become the policeman of the world, at great expense in blood and treasure. Most of these actions end up failing to deliver the advertised outcome. As history shows, most of the foreign leaders we have entangled ourselves with, later ended up becoming our enemies. Hussein, Bin Laden and Noriega are all classic examples of the principle of unintended consequences involving foreign entanglements, or "blowback" as the CIA calls it today.
This empire is not only immoral, and unnecessary, it's also financially unsustainable.
The number of bases we have on foreign soil totals over 700. This is wrong for several reasons. First, it violates the sovereignty of those nations in which we have bases. Second, it is incredibly expensive. Third, it actually reduces our security, which is the opposite of the advertised benefit. It does so by instilling resentment among the populations of the host countries, and also by spreading us so thin across the globe, that we might be unable to protect ourselves from a serious attack at home.
Some empire supporters might characterize this as being anti-defense or anti-military, however nothing could be farther from the truth. Having served in the Air Force for 10 years, I am very pro defense, and pro military. My view is that our military should be used to defend America, and not be used to defend or police the rest of the world. The benefit from such a policy shift would be twofold. It would increase our economic security because of the greatly reduced cost, and increase our physical security by focusing on our home soil, rather than being spread so thinly across the globe as we are today.