Does the United States now follow a moral idealist or a political realist approach in international affairs?
I disagree. I believe we have been practicing "realpolitik" since the beginning of World War II, and that, while we speak of lofty foreign policy ideals, it is usually just hollow rhetoric.
To argue that President Bush was an idealist by going into Afghanistan and Iraq, we would have to believe his motivation was purely based on establishing democracies in those countries, and nation-building their economies. We would have to overlook our military goal of destroying al-Qaeda (a realist response, based on a direct attack), removing Saddam Hussein (viewed, rightly or wrongly, as a threat to interests and allies) and securing a source of the oil on which our economy is based (can't get more realist than that).
In doing so, we have decided practicality-based policies from Predator-drone assassinations, to extraordinary rendition (exporting detainees to countries where we know they will be tortured), to torture (Guantanamo Bay Prison), all in the name of protecting the nation, and out of the realm of idealism altogether. President Bush did say publicly though that "We do not torture". It just wasn't true. So it's important to recognize and acknowledge the difference between political speech and the actions of realpolitik.
In my opinion, the United States has not been pursuing a realist approach for quite some time. We have been purusing more of an idealist approach at least since the time of 9/11.
Although Pres. Bush said he wanted to be more of a realist, his actions in Afghanistan and Iraq have not gone along with that point of view. It is hard to argue that our continued presence in those countries is vital to our national security. Instead, it seems as if we are trying to "rehabilitate" those countries the way that idealists would prefer.
Of course, we do not always act in this way. For example, we have not been all that interested in intervening in Darfur even though idealism might say that we should.