Duke Constantine Frederick Peter of Oldenburg was a Duke of the House of Oldenburg. He was the grandfather of Duke Peter of Oldenburg as well as grandfather of Grand Duke Nicholas Nikolaevich, General of the Imperial Russian Army during World War I. His great-great-grandson, Nicholas Romanov, was the President of the Romanov Family Association until his death in 2014.

Grand Duke Nicholas Nikolaevich of Russia was a Russian general in World War I (1914–1918). The son of Grand Duke Nicholas Nikolaevich of Russia (1831–1891), and a grandson of Emperor Nicholas I of Russia, he was commander in chief of the Imperial Russian Army units on the main front in the first year of the war, during the reign of his first cousin once removed, Nicholas II. Although held in high regard by Paul von Hindenburg, he struggled with the colossal task of leading Russia's war effort against Germany, including strategy, tactics, logistics and coordination with the government. After the Gorlice–Tarnów offensive in 1915, Tsar Nicholas replaced the Grand Duke as commander-in-chief of the army. He later was a successful commander-in-chief in the Caucasus region. He was briefly recognized as emperor in 1922 in areas controlled by the White movement in the Russian Far East.