Russian Empire
Though the Empire was only officially proclaimed by Tsar Peter I following the Treaty of Nystad (1721), some historians would argue that it was truly born either when Ivan III of Russia conquered Veliky Novgorod in 1478, or when Ivan the Terrible conquered the Khanate of Kazan in 1552. According to another point of view, the term "Tsardom", which was used after the coronation of Ivan IV in 1547, was already a contemporary Russian word for empire.
Duke Constantine Petrovich of Oldenburg
During the 18th Century, the Dukes of Holstein-Gottorp gained influence through a carefully planned series of marital alliances with the royal houses of Germany, Denmark, Sweden, Norway and Prussia. The childless Empress Elizabeth of Russia proclaimed her nephew, Charles Peter Ulrich of Holstein-Gottorp heir to the throne and when he became Tsar Peter III of Russia in 1762, the Holstein-Gottorps, themselves a cadet branch of the House of Oldenburg, became the Imperial house of Russia, which they ruled until 1917 under the name of Romanov.