In this case....die is the 'the' world....in German.
Realpolitik is this charming world which has to deal with German perceptions of an agenda in political circles....that has no connection to ethics, morals, or obligations. It comes out of the 1800s and the Prussian period where politics was concluded by an inner-circle (mostly the Kaiser and the military leadership), and it dealt with the power of authority.
Some Germans would argue that Realpolitik has evolved or changed into something totally different here in 2016. Some Germans would say that nothing much has changed in Berlin politics in the last hundred years.
If the topic comes up in some conversation between you and a couple of Germans.....it's obvious that your conversation has drifted out of safe territory of soccer, cooking, automobiles, or lawn maintenance. This might be the point where you pay for your beer and exit the pub.