I think the genealogy of the Oldenburg family is soooo interesting.
Beginning as the rulers of a tiny piece of land in the HRE, by a clever marriage to a Holstein princess distantly related to the Danish royal house, they eventually ended up inheriting the crowns of Denmark, Norway and Sweden. A younger line kept Oldenburg and Delmenhorst in HRE...and later on junior princes of the Danish royal house became Kings of Greece and Czar of Russia.
One geneartion I find particularly interesting is Karl Peter Ulrich (1728-1762). In addition to his own Oldenburger claims via the paternal line, his grandmother Hedwige Sophie Auguste of Sweden was the sister of Queen Ulrika Eleonora of Sweden, so he was intended to inherit the Swedish throne if Ulrika had no heirs. However, his own mother was Anna, Grand Duchess of Russia, and he ended up succeeding her sister the Czarina Elizabeth as Czar Peter III; apparently he cherished hopes of uniting his inherited Russia with Sweden and his own patrimony, the Duchy of Holstein-Gottorp.
For whatever reason, he was rejected as King by the Swedes, and they made his cousin Adolf Friedrich their King instead.
Adolf Friderich's sister Johanna married Christian August, Furst von Anhalt-Zerbst, a general in the Prussian army and the ruler of a tiny tiny principality in Germany. They had a number of children, one of which was a young princess Sophie of Anhalt-Zerbst, who was chosen by the Czarina Elizabeth as a suitable bride for her heir Peter. Later on, she became the Czarina Catherine in her own right.
I had never realised how closely Catherine and Peter were related, much less that she was of so exalted a lineage as the Holstein-Gottorps and the future Swedish royal house. Most books always stress how her family were of lowly rank, unimportant, with no money, and make it look like she had won the jackpot by making a marriage with the young Duke of Holstein-Gottorp, future Czar.
When in reality, they were cousins, so of equal rank and birth, and (I guess) his marriage to her was most likely intended to double up on his own claims to the Swedish throne, ie, in case Adolf Friedrich died he was have a good clam not only in his own right but in right of his wife, Adolf's niece.
Anyways, their son Paul I ceded his lands in Holstein to Denmark, and received Oldenburg and Delmenhorst instead, although he didn't keep it and gave it to a cousin, who I believe in the ancestor of the current Duke Anthon Gunther.
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