William I of Württemberg
On 24 January 1816, in Saint Petersburg, he married his first cousin, Grand Duchess Catherine Pavlovna of Russia (1788–1819), daughter of Emperor Paul I of Russia (1754–1801) and Princess Sophie Dorothea of Württemberg (1759–1828). The couple had two children:
Duke Constantine Petrovich of Oldenburg
Tsar Peter III of Russia married on 21 August 1745 to his second cousin, the Prussian princess Sophie Friederike Auguste von Anhalt-Zerbst-Dornburg, better known as Catherine II or Catherine the Great. The couple had one son, Paul, who, after the death of Empress Catherine in 1796, ruled as Emperor Paul I of Russia until his assassination in 1801. Paul and his wife, Maria Feodorovna, Duchess of Württemberg, had 10 children. Their eldest son ruled as Emperor Alexander I of Russia between 1801 and 1825, the period of the Napoleonic Wars.