George was born in Copenhagen, and was the second son of Prince Christian of Schleswig-Holstein-Sonderburg-Glücksburg and Louise of Hesse-Kassel.[3] Although his full name was Prince Christian Wilhelm Ferdinand Adolf Georg of Schleswig-Holstein-Sonderburg-Glücksburg, until his accession in Greece, he was known as Prince Vilhelm (William), the namesake of his paternal and maternal grandfathers,[4] Frederick William, Duke of Schleswig-Holstein-Sonderburg-Glücksburg, and Prince William of Hesse-Kassel. In 1852, George's father was designated the heir presumptive to the childless King Frederick VII of Denmark, and the family became princes and princesses of Denmark. George's siblings were Frederick (who succeeded their father as King of Denmark), Alexandra (who became queen consort of Edward VII of the United Kingdom and the mother of King George V), Dagmar (who, as Empress Maria Feodorovna, was consort of Alexander III of Russia and the mother of Tsar Nicholas II), Thyra (who married Prince Ernest Augustus, 3rd Duke of Cumberland and Teviotdale) and Valdemar.[3] George embarked on a career in the Royal Danish Navy, and enrolled as a naval cadet along with his elder brother Frederick. While Frederick was described as "quiet and extremely well-behaved", George was "lively and full of pranks".[5]

George I (Greek: Γεώργιος Α΄, Βασιλεύς των Ελλήνων, Geórgios Α΄, Vasiléfs ton Ellínon; born Prince William of Schleswig-Holstein-Sonderburg-Glücksburg; 24 December 1845 – 18 March 1913[1]) was King of Greece from 1863 to 1913. Originally a Danish prince, George was born in Copenhagen, and seemed destined for a career in the Royal Danish Navy. He was only 17 years old when he was elected king by the Greek National Assembly, which had deposed the unpopular former king Otto. His nomination was both suggested and supported by the Great Powers: the United Kingdom of Great Britain and Ireland, the Second French Empire and the Russian Empire. He married the Russian grand duchess Olga Constantinovna of Russia, and became the first monarch of a new Greek dynasty. Two of his sisters (Alexandra and Dagmar) married into the British and Russian royal families. Edward VII and Tsar Alexander III were his brothers-in-law. George's reign of almost 50 years (the longest in modern Greek history) was characterized by territorial gains as Greece established its place in pre-World War I Europe. Britain ceded the Ionian Islands peacefully, while Thessaly was annexed from the Ottoman Empire after the Russo-Turkish War (1877–1878). Greece was not always successful in its expansionist ambitions; it was defeated in the Greco-Turkish War (1897). During the First Balkan War, after Greek troops had occupied much of Greek Macedonia, George was assassinated in Thessaloniki. In sharp contrast to his own reign, the reigns of his successors proved short and insecure.