direct border to the mighty Frankish Empire posed a strategic threat to Danish independence. In fact, many historians have seen the construction of the first border fortifications, known as the Danevirke, as the first proof of the beginning of an independent Danish state. The border was challenged from both sides; Charlemagne sought to conquer Denmark, and Danish kings supported Saxons wishing to cast off the Frankish rule. Despite minor wars, a border was relatively quickly established at the River Eider. This border, which Denmark later considered to be its "natural border" is exemplified in a stone once set in the walls of Rendsburg (), a city on the border between Schleswig and Holstein. The stone had the following inscription: Eidora Terminus Imperii Romani (The Roman Empire ends at the Eider) Valdemar the Victorious During the early Middle Ages, a number of nobles from Holstein tried to expand their influence across the border into Schleswig. Most notably of these were the Counts of Schauenburg and Holstein ruling Holstein. The border was also challenged by a number of Danish kings, most notably King Valdemar the Victorious who sought to obtain control of Northern Germany, then a part of the Holy Roman Empire. 1214 marked an important step in this process, when the Holy Roman Emperor Frederick II issued a Golden Bull surrendering all imperial territories north of the rivers Elbe and Elde to Valdemar the Victorious. An action provoking many Holsatian nobles. In 1223, the fates were reversed when Henry I, Count of Schwerin (a.k.a. Heinrich der Schwarze) abducted Valdemar and his eldest son and imprisoned them in the Waldemarturm of Dannenberg Castle. Henry demanded that Valdemar should surrender all of his conquests in the Holy Roman Empire and instead swear allegiance to the Holy Roman Emperor. In fact, Emperor Frederick II tried to intervene and arrange Valdemar's release, but he was rebuffed by Count Henry. Henry's terms were flatly refused by the Danish envoys sent to negotiate the release of their king, and Danish nobles decided to attack Henry. The war ended in 1225 in Danish defeat. Valdemar was now forced to not only surrender his conquests in the Holy Roman Empire but also to take an oath not to seek revenge. Henry released him in 1226 and Valdemar immediately appealed to Pope Honorius III to have his oath declared void, a request granted by the Pope. In 1226, Valdemar returned