Holstein is the region between the rivers Elbe and Eider. It is the southern half of Schleswig-Holstein, the northernmost state of Germany. Holstein once existed as the German County of Holstein, the later Duchy of Holstein, and was the northernmost territory of the Holy Roman Empire. The history of Holstein is closely intertwined with the history of the Da…Holstein is the region between the rivers Elbe and Eider. It is the southern half of Schleswig-Holstein, the northernmost state of Germany. Holstein once existed as the German County of Holstein, the later Duchy of Holstein, and was the northernmost territory of the Holy Roman Empire. The history of Holstein is closely intertwined with the history of the Danish Duchy of Schleswig. The capital of Holstein is Kiel.