The monarchy was strengthened by the taking back of former crown land which had been given to nobles, and within twenty years Denmark had regained its former lands, with German forces driven back across the Ejder The only loss was Estonia, a Danish possession since 1219, which was sold to the Order of Teutonic Knights. In 1361, the buoyant king attacked and conquered Gotland, much to the annoyance of the Hanseatic League, who were using it as a Baltic trading base. A number of antiDanish alliances sprang up and tfie country was slowly plundered until peace was agreed in 1370 under the Treaty of Stralsund. This guaranteed trade for the Hanseatic partners by granting them control of castles along the west coast of SkSne for fifteen years. It also laid down that the election of the Danish monarch had to be approved by the Hanseatic League the peak of their power. ¦ The Kalmar Union Valdemar's daughter Margrethe forced the election of her fiveyearold son, Olav, as king in 1380, installing herself as regent After his untimely death after only a sevenyear reign, Margrethe became queen of Denmark and Nonway, and later of Sweden as well the first ruler of a united Scandinavia. In 1397, a formal document the Kalmar Union, set out the rules of the union of the countries, which allowed for a Scandinavian federation under the same monarch and foreign policy, within which each country had its own internal legislation. It became evident that Denmark was to be the dominant partner within the union when Margrethe placed Danish nobles in civic positions in Nonway and Sweden but failed to reciprocate with Swedes and Norwegians in Denmark. Erik VII ("of Pomerania") became king in 1396, and was determined to remove the Counts of Holstein who had taken possession of Schleswig. In 1413 he persuaded a meeting of the Danehof to declare the whole of Schleswig to be crown property, and three years later war broke out with the Germaninfluenced nobility of the region. Initially the Hanseatic League supported the king, unhappy with the Holstein privateers who were interfering with their trade But Erik also introduced important economic reforms within Denmark, ensuring that foreign goods reached Danish people through Danish merchants instead of coming directly from Hanseatic traders. This led to a war with the League, after which, in 1429, Erik imposed tne Sound toll on shipping passing through the narrow strip of sea off the coast of Helsinger. The conflicts with the Holsteiners and the Hanseatic League had, however, badly drained financial resources. Denmark still relied on hired armies to do its fighting, and the burden of taxation had caused widespread dissatisfaction, particularly in Sweden. With the Holstein forces gaining ground in Jutland, Erik fled to Gotland and in 1439 Swedish and Danish nobles elected in his place Christoffer III, who acquiesced in the nobles' demands and ensured peace with the Hanseatic League by granting them exemption from the Sound toll.