The kings of Denmark were granted Holstein as an imperial fief by the Holy Roman Emperor Frederick III in 1474.

According to the 1720 Treaty of Frederiksborg, Swedish support for Gottorp ceased, making it impossible for the dukes to regain their lost territories in Schleswig and prolonging their feud with the king of Denmark.

But from this marriage was born Charles Peter Ulrich, who succeeded to Holstein-Gottorp in 1739, and became an heir to the Russian throne according to the will of Catherine I and especially upon the accession of his childless aunt Elizabeth in 1741.

This move angered Russian opinion, since it was considered a betrayal of Russia's sacrifices in the war, as well as placing national interests in jeopardy.

At the same time, the Danish army had hastily moved across the border into Mecklenburg, to avoid an invasion of Holstein, and prepared for battle.

With the 1773 Treaty of Tsarskoye Selo, she agreed to cede the territorial claims of her son to the Holstein-Gottorp lands still held by Denmark and to cede the part of Duchy, held by her husband, obtaining in exchange the German countships of Oldenburg and Delmenhorst, elevated in 1776 into the duchy of Oldenburg within the Holy Roman Empire.

The dynastic policy of the dukes of Holstein-Gottorp resulted in its cadet branch, the Swedish line, ruling Sweden from 1751 until 1818 and Norway from 1814 to 1818.

If, however, all marriages deemed morganatic by Russian Imperial standards were also non-dynastic for the Gottorp succession, the genealogically senior Holstein-Gottorp dynast would be Christian, Duke of Oldenburg, current head of the branch descending from Christian August of Holstein-Gottorp, Prince of Eutin, the younger brother of Duke Frederick IV.

Either way, the king of Denmark exercised sovereignty in the duchies and provided financial support to the cadet Schleswig-Holstein branches of the House of Oldenburg.

The claim to Holstein inherited by Emperor Paul I from Peter III was exchanged in 1773 for the Danish kings' duchy of Oldenburg (residual succession rights being retained), the rulers of which lost sovereignty there in 1918.

Danish monarchs continued to use their traditional ducal titles in pretence until the death of king Frederik IX of Denmark in 1972.

In 1920, Northern Schleswig was returned to Danish rule after a plebiscite, the remainder of the former duchies remains part of Germany.